diff --git a/Changes b/Changes
index fdf8905a7..643c23767 100644
--- a/Changes
+++ b/Changes
@@ -6,6 +6,8 @@ The contributors that suggested a given feature are shown in []. Thanks!
 
 ***   Add +verilator+error+limit to see more assertion errors. [Peter Monsson]
 
+***   Support string.toupper and string.tolower.
+
 ***   Support $rewind and $ungetc.
 
 ****  Add -Wpedantic for compliance testing.
diff --git a/include/verilated.cpp b/include/verilated.cpp
index efb54dcae..dc1528458 100644
--- a/include/verilated.cpp
+++ b/include/verilated.cpp
@@ -1754,7 +1754,22 @@ const char* vl_mc_scan_plusargs(const char* prefixp) VL_MT_SAFE {
 }
 
 //===========================================================================
-// Heavy functions
+// Heavy string functions
+
+std::string VL_TOLOWER_NN(const std::string& ld) VL_MT_SAFE {
+    std::string out = ld;
+    for (std::string::iterator it = out.begin(); it != out.end(); ++it) {
+        *it = tolower(*it);
+    }
+    return out;
+}
+std::string VL_TOUPPER_NN(const std::string& ld) VL_MT_SAFE {
+    std::string out = ld;
+    for (std::string::iterator it = out.begin(); it != out.end(); ++it) {
+        *it = toupper(*it);
+    }
+    return out;
+}
 
 std::string VL_CVT_PACK_STR_NW(int lwords, WDataInP lwp) VL_MT_SAFE {
     // See also _VL_VINT_TO_STRING
diff --git a/include/verilated_heavy.h b/include/verilated_heavy.h
index b071e5af4..7e0e9dd7a 100644
--- a/include/verilated_heavy.h
+++ b/include/verilated_heavy.h
@@ -61,6 +61,8 @@ inline std::string VL_REPLICATEN_NNI(int obits,int lbits,int rbits,
 }
 
 inline IData VL_LEN_IN(const std::string& ld) { return ld.length(); }
+extern std::string VL_TOLOWER_NN(const std::string& ld);
+extern std::string VL_TOUPPER_NN(const std::string& ld);
 
 extern IData VL_FOPEN_NI(const std::string& filename, IData mode) VL_MT_SAFE;
 extern void VL_READMEM_N(bool hex, int width, int depth, int array_lsb,
diff --git a/src/V3AstNodes.h b/src/V3AstNodes.h
index 9eb73f70a..4dc7150ca 100644
--- a/src/V3AstNodes.h
+++ b/src/V3AstNodes.h
@@ -4532,6 +4532,32 @@ public:
     virtual string emitVerilog() { return "%f$atanh(%l)"; }
     virtual string emitC() { return "atanh(%li)"; }
 };
+class AstToLowerN : public AstNodeUniop {
+    // string.tolower()
+public:
+    AstToLowerN(FileLine* fl, AstNode* lhsp) : AstNodeUniop(fl, lhsp) {
+        dtypeSetString(); }
+    ASTNODE_NODE_FUNCS(ToLowerN)
+    virtual void numberOperate(V3Number& out, const V3Number& lhs) { out.opToLowerN(lhs); }
+    virtual string emitVerilog() { return "%l.tolower()"; }
+    virtual string emitC() { return "VL_TOLOWER_NN(%li)"; }
+    virtual bool cleanOut() const { return true; }
+    virtual bool cleanLhs() const { return true; }
+    virtual bool sizeMattersLhs() const { return false; }
+};
+class AstToUpperN : public AstNodeUniop {
+    // string.toupper()
+public:
+    AstToUpperN(FileLine* fl, AstNode* lhsp) : AstNodeUniop(fl, lhsp) {
+        dtypeSetString(); }
+    ASTNODE_NODE_FUNCS(ToUpperN)
+    virtual void numberOperate(V3Number& out, const V3Number& lhs) { out.opToUpperN(lhs); }
+    virtual string emitVerilog() { return "%l.toupper()"; }
+    virtual string emitC() { return "VL_TOUPPER_NN(%li)"; }
+    virtual bool cleanOut() const { return true; }
+    virtual bool cleanLhs() const { return true; }
+    virtual bool sizeMattersLhs() const { return false; }
+};
 
 //======================================================================
 // Binary ops
diff --git a/src/V3Number.cpp b/src/V3Number.cpp
index 8acebbe45..67d11248d 100644
--- a/src/V3Number.cpp
+++ b/src/V3Number.cpp
@@ -2074,6 +2074,25 @@ V3Number& V3Number::opReplN(const V3Number& lhs, uint32_t rhsval) {
     }
     return setString(out);
 }
+V3Number& V3Number::opToLowerN(const V3Number& lhs) {
+    NUM_ASSERT_OP_ARGS1(lhs);
+    NUM_ASSERT_STRING_ARGS1(lhs);
+    std::string out = lhs.toString();
+    for (std::string::iterator it = out.begin(); it != out.end(); ++it) {
+        *it = tolower(*it);
+    }
+    return setString(out);
+}
+V3Number& V3Number::opToUpperN(const V3Number& lhs) {
+    NUM_ASSERT_OP_ARGS1(lhs);
+    NUM_ASSERT_STRING_ARGS1(lhs);
+    std::string out = lhs.toString();
+    for (std::string::iterator it = out.begin(); it != out.end(); ++it) {
+        *it = toupper(*it);
+    }
+    return setString(out);
+}
+
 V3Number& V3Number::opEqN(const V3Number& lhs, const V3Number& rhs) {
     NUM_ASSERT_OP_ARGS2(lhs, rhs);
     NUM_ASSERT_STRING_ARGS2(lhs, rhs);
diff --git a/src/V3Number.h b/src/V3Number.h
index 65aeef905..6a8af81d6 100644
--- a/src/V3Number.h
+++ b/src/V3Number.h
@@ -288,6 +288,8 @@ public:
     V3Number& opSel     (const V3Number& lhs, uint32_t msbval, uint32_t lsbval);
     V3Number& opSelInto (const V3Number& lhs, const V3Number& lsb, int width);
     V3Number& opSelInto (const V3Number& lhs, int lsbval, int width);
+    V3Number& opToLowerN(const V3Number& lhs);
+    V3Number& opToUpperN(const V3Number& lhs);
     V3Number& opCond    (const V3Number& lhs, const V3Number& if1s, const V3Number& if0s);
     V3Number& opCaseEq  (const V3Number& lhs, const V3Number& rhs);
     V3Number& opCaseNeq (const V3Number& lhs, const V3Number& rhs);
diff --git a/src/V3Width.cpp b/src/V3Width.cpp
index 532639c81..85c6d8a7f 100644
--- a/src/V3Width.cpp
+++ b/src/V3Width.cpp
@@ -442,6 +442,18 @@ private:
             }
         }
     }
+    virtual void visit(AstToLowerN* nodep) {
+        if (m_vup->prelim()) {
+            iterateCheckString(nodep, "LHS", nodep->lhsp(), BOTH);
+            nodep->dtypeSetString();
+        }
+    }
+    virtual void visit(AstToUpperN* nodep) {
+        if (m_vup->prelim()) {
+            iterateCheckString(nodep, "LHS", nodep->lhsp(), BOTH);
+            nodep->dtypeSetString();
+        }
+    }
     virtual void visit(AstReplicate* nodep) {
         // IEEE-2012 Table 11-21:
         //   LHS, RHS is self-determined
@@ -1804,8 +1816,26 @@ private:
         } else if (nodep->name() == "realtoa") {
             methodOkArguments(nodep, 1, 1);
             replaceWithSFormat(nodep, "%g"); VL_DANGLING(nodep);
-        } else {
+        } else if (nodep->name() == "tolower") {
+            methodOkArguments(nodep, 0, 0);
+            AstNode* newp = new AstToLowerN(nodep->fileline(), nodep->fromp()->unlinkFrBack());
+            nodep->replaceWith(newp); nodep->deleteTree(); VL_DANGLING(nodep);
+        } else if (nodep->name() == "toupper") {
+            methodOkArguments(nodep, 0, 0);
+            AstNode* newp = new AstToUpperN(nodep->fileline(), nodep->fromp()->unlinkFrBack());
+            nodep->replaceWith(newp); nodep->deleteTree(); VL_DANGLING(nodep);
+        } else if (nodep->name() == "atobin"
+                   || nodep->name() == "atohex"
+                   || nodep->name() == "atoi"
+                   || nodep->name() == "atooct"
+                   || nodep->name() == "atoreal"
+                   || nodep->name() == "compare"
+                   || nodep->name() == "icompare"
+                   || nodep->name() == "getc"
+                   || nodep->name() == "putc") {
             nodep->v3error("Unsupported: built-in string method "<<nodep->prettyNameQ());
+        } else {
+            nodep->v3error("Unknown built-in string method "<<nodep->prettyNameQ());
         }
     }
 
diff --git a/test_regress/t/t_string_type_methods.v b/test_regress/t/t_string_type_methods.v
index fb71cb06e..85eadb79f 100644
--- a/test_regress/t/t_string_type_methods.v
+++ b/test_regress/t/t_string_type_methods.v
@@ -20,11 +20,11 @@ module t (/*AUTOARG*/
    // Check constification
    initial begin
       s="1234"; `checkh(s.len(),4);
+      s="ab7CD"; `checks(s.toupper(), "AB7CD");
+      s="ab7CD"; `checks(s.tolower(), "ab7cd");
 `ifndef VERILATOR
       s="1234"; s.putc(2, "z"); `checks(s, "12z4");
       s="1234"; `checkh(s.getc(2), "3");
-      s="abCD"; `checks(s.toupper(), "ABCD");
-      s="abCD"; `checks(s.tolower(), "abcd");
       s="b"; if (s.compare("a") <= 0) $stop;
       s="b"; if (s.compare("b") != 0) $stop;
       s="b"; if (s.compare("c") >= 0) $stop;
@@ -42,6 +42,9 @@ module t (/*AUTOARG*/
       s.octtoa(123); `checks(s, "173");
       s.bintoa(123); `checks(s, "1111011");
       s.realtoa(1.23); `checks(s, "1.23");
+      s = "bAr";
+      s = s.toupper; `checks(s, "BAR");
+      s = s.tolower; `checks(s, "bar");
    end
 
    // Check runtime
@@ -54,40 +57,48 @@ module t (/*AUTOARG*/
       else if (cyc==1) begin
          `checkh(s.len(),4);
       end
-`ifndef VERILATOR
       else if (cyc==2) begin
+`ifndef VERILATOR
          s.putc(2, "z");
+`endif
       end
       else if (cyc==3) begin
+`ifndef VERILATOR
          `checks(s, "12z4");
          `checkh(s.getc(2), "z");
-         s="abCD";
+`endif
+         s="ab3CD";
       end
       else if (cyc==4) begin
-         `checks(s.toupper(), "ABCD");
-         `checks(s.tolower(), "abcd");
+         `checks(s.toupper(), "AB3CD");
+         `checks(s.tolower(), "ab3cd");
          s="b";
       end
       else if (cyc==5) begin
+`ifndef VERILATOR
          if (s.compare("a") <= 0) $stop;
          if (s.compare("b") != 0) $stop;
          if (s.compare("c") >= 0) $stop;
          if (s.icompare("A") < 0) $stop;
          if (s.icompare("B") != 0) $stop;
          if (s.icompare("C") >= 0) $stop;
+`endif
          s="101";
       end
       else if (cyc==7) begin
+`ifndef VERILATOR
          `checkh(s.atoi(), 'd101);
          `checkh(s.atohex(), 'h101);
          `checkh(s.atooct(), 'o101);
          `checkh(s.atobin(), 'b101);
+`endif
          s="1.23";
       end
       else if (cyc==8) begin
+`ifndef VERILATOR
          `checkg(s.atoreal(), 1.23);
-      end
 `endif
+      end
       else if (cyc==9) begin
          s.itoa(123);
       end