diff --git a/Changes b/Changes
index be76d962d..76d93f963 100644
--- a/Changes
+++ b/Changes
@@ -10,6 +10,8 @@ indicates the contributor was also the author of the fix; Thanks!
 
 ***  Allow multiple .v files on command line.  [Stefan Thiede]
 
+***  Convert re-defining macro error to warning.  [Stefan Thiede]
+
 **** Fix genvar to be signed, so "< 0" works properly.  [Niranjan Prabhu]
 
 **** Fix assignments to inputs inside functions/tasks.  [Patricio Kaplan]
diff --git a/bin/verilator b/bin/verilator
index c4e995258..6565278f7 100755
--- a/bin/verilator
+++ b/bin/verilator
@@ -1695,6 +1695,23 @@ not really needed.  The best solution is to insure that each module is in a
 unique file by the same name.  Otherwise, make sure all library files are
 read in as libraries with -v, instead of automatically with -y.
 
+=item REDEFMACRO
+
+Warns that you have redefined the same macro with a different value, for
+example:
+
+    `define MACRO def1
+    //...
+    `define MACRO otherdef
+
+The best solution is to use a different name for the second macro.  If this
+is not possible, add a undef to indicate the code is overriding the value:
+
+    `define MACRO def1
+    //...
+    `undef MACRO
+    `define MACRO otherdef
+
 =item STMTDLY
 
 Warns that you have a statement with a delayed time in front of it, for
diff --git a/src/V3Error.h b/src/V3Error.h
index bfae5fa84..c40f0aca5 100644
--- a/src/V3Error.h
+++ b/src/V3Error.h
@@ -53,6 +53,7 @@ public:
 	IMPLICIT,	// Implicit wire
 	IMPURE,		// Impure function not being inlined
 	MULTIDRIVEN,	// Driven from multiple blocks
+	REDEFMACRO,	// Redefining existing define macro
 	UNDRIVEN,	// No drivers
 	UNOPT,		// Unoptimizable block
 	UNOPTFLAT,	// Unoptimizable block after flattening
@@ -78,7 +79,7 @@ public:
 	    "BLKANDNBLK",
 	    "CASEINCOMPLETE", "CASEOVERLAP", "CASEX", "CMPCONST",
 	    "COMBDLY", "STMTDLY", "GENCLK", "IMPLICIT", "IMPURE",
-	    "MULTIDRIVEN",
+	    "MULTIDRIVEN", "REDEFMACRO",
 	    "UNDRIVEN", "UNOPT", "UNOPTFLAT", "UNSIGNED", "UNUSED",
 	    "VARHIDDEN", "WIDTH",
 	    " MAX"
diff --git a/src/V3PreProc.cpp b/src/V3PreProc.cpp
index 0c4447293..a2130a850 100644
--- a/src/V3PreProc.cpp
+++ b/src/V3PreProc.cpp
@@ -207,8 +207,8 @@ void V3PreProcImp::define(FileLine* fl, const string& name, const string& value,
     UINFO(4,"DEFINE '"<<name<<"' as '"<<value<<"' params '"<<params<<"'"<<endl);
     if (defExists(name)) {
 	if (!(defValue(name)==value && defParams(name)==params)) {  // Duplicate defs are OK
-	    fl->v3error("Define already exists: "<<name);
-	    defFileline(name)->v3error("Previous definition is here.");
+	    fl->v3warn(REDEFMACRO,"Redefining existing define: "<<name<<", with different value: "<<value<<" "<<params);
+	    defFileline(name)->v3warn(REDEFMACRO,"Previous definition is here, with value: "<<defValue(name)<<" "<<defParams(name));
 	}
 	undef(name);
     }
diff --git a/test_regress/t/t_pp_dupdef.pl b/test_regress/t/t_pp_dupdef.pl
new file mode 100755
index 000000000..1ecdda807
--- /dev/null
+++ b/test_regress/t/t_pp_dupdef.pl
@@ -0,0 +1,16 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("./driver.pl", @ARGV, $0); die; }
+# $Id$
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2003 by Wilson Snyder. This program is free software; you can
+# redistribute it and/or modify it under the terms of either the GNU
+# General Public License or the Perl Artistic License.
+
+compile (
+	 v_flags2 => ["-Wno-REDEFMACRO"],
+	 ) if $Last_Self->{v3};
+
+ok(1);
+1;
+
diff --git a/test_regress/t/t_pp_dupdef.v b/test_regress/t/t_pp_dupdef.v
new file mode 100644
index 000000000..8de2b8abe
--- /dev/null
+++ b/test_regress/t/t_pp_dupdef.v
@@ -0,0 +1,17 @@
+// $Id$
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed into the Public Domain, for any use,
+// without warranty, 2008 by Wilson Snyder.
+
+module t;
+
+`define DUP fred
+`define DUP barney
+
+`define DUPP paramed(x) (x)
+`define DUPP paramed(x,z) (x*z)
+   
+     initial $stop; // Should have failed
+
+endmodule
diff --git a/test_regress/t/t_pp_dupdef_bad.pl b/test_regress/t/t_pp_dupdef_bad.pl
new file mode 100755
index 000000000..bae7876fa
--- /dev/null
+++ b/test_regress/t/t_pp_dupdef_bad.pl
@@ -0,0 +1,26 @@
+#!/usr/bin/perl
+if (!$::Driver) { use FindBin; exec("./driver.pl", @ARGV, $0); die; }
+# $Id$
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2008 by Wilson Snyder. This program is free software; you can
+# redistribute it and/or modify it under the terms of either the GNU
+# General Public License or the Perl Artistic License.
+
+top_filename("t/t_pp_dupdef.v");
+
+compile (
+	 v_flags2 => ["--lint-only"],
+	 fails=>1,
+	 expect=>
+'%Warning-REDEFMACRO: t/t_pp_dupdef.v:\d+: Redefining existing define: DUP, with different value: barney 
+%Warning-REDEFMACRO: Use .* to disable this message.
+%Warning-REDEFMACRO: t/t_pp_dupdef.v:\d+: Previous definition is here, with value: fred 
+%Warning-REDEFMACRO: t/t_pp_dupdef.v:\d+: Redefining existing define: DUPP, with different value: .*
+%Warning-REDEFMACRO: t/t_pp_dupdef.v:\d+: Previous definition is here, with value: .*
+%Error: Exiting due to.*',
+	 ) if $Last_Self->{v3};
+
+ok(1);
+1;
+