diff --git a/docs/changes.txt b/docs/changes.txt
index 0beea8ce8a..2aa5da4f36 100644
--- a/docs/changes.txt
+++ b/docs/changes.txt
@@ -32,6 +32,7 @@ Changes in behaviour which may result in build errors
 
 All:
 
+- Add "%z" support to printf()-like functions like wxString::Format() (RIVDSL).
 - Add wxPowerResourceBlocker (Tobias Taschner).
 - Add wxApp::StoreCurrentException() and RethrowStoredException() and implement
   their functionality by default when using C++11 compiler.
diff --git a/src/common/strvararg.cpp b/src/common/strvararg.cpp
index aa698a58b3..0196e06ee9 100644
--- a/src/common/strvararg.cpp
+++ b/src/common/strvararg.cpp
@@ -198,6 +198,17 @@ public:
 
                 switch ( *format )
                 {
+#ifdef __VISUALC__
+                    case 'z':
+                        // Used for size_t printing (e.g. %zu) and is in C99,
+                        // but is not portable, MSVC uses 'I' with the same
+                        // meaning.
+                        ChangeFmtChar('I');
+                        format++;
+                        size = Size_Default;
+                        break;
+#endif // __VISUALC__
+
                     case 'h':
                         size = Size_Short;
                         format++;
@@ -341,6 +352,18 @@ private:
         *(m_fmtLast++) = ch;
     }
 
+    // change a character
+    void ChangeFmtChar(CharType ch)
+    {
+        if ( m_fmtOrig )
+        {
+            // so far we haven't translated anything yet
+            CopyAllBefore();
+        }
+
+        *m_fmtLast = ch;
+    }
+
     void CopyAllBefore()
     {
         wxASSERT_MSG( m_fmtOrig && m_fmt.data() == NULL, "logic error" );
diff --git a/tests/strings/vararg.cpp b/tests/strings/vararg.cpp
index b733516874..cc5d4d51c4 100644
--- a/tests/strings/vararg.cpp
+++ b/tests/strings/vararg.cpp
@@ -36,6 +36,7 @@ private:
     CPPUNIT_TEST_SUITE( VarArgTestCase );
         CPPUNIT_TEST( StringPrintf );
         CPPUNIT_TEST( CharPrintf );
+        CPPUNIT_TEST( SizetPrintf );
 #if wxUSE_STD_STRING
         CPPUNIT_TEST( StdString );
 #endif
@@ -49,6 +50,7 @@ private:
 
     void StringPrintf();
     void CharPrintf();
+    void SizetPrintf();
 #if wxUSE_STD_STRING
     void StdString();
 #endif
@@ -138,6 +140,24 @@ void VarArgTestCase::CharPrintf()
     CPPUNIT_ASSERT_EQUAL( "value is 240 (int)", s );
 }
 
+void VarArgTestCase::SizetPrintf()
+{
+    size_t  i =  1;
+    ssize_t j = -2;
+
+    CPPUNIT_ASSERT_EQUAL
+        (
+            "size_t=1 ssize_t=-2",
+            wxString::Format("size_t=%zu ssize_t=%zd", i, j)
+        );
+
+    CPPUNIT_ASSERT_EQUAL
+        (
+            "size_t=0xA0",
+            wxString::Format("size_t=0x%zX", static_cast<size_t>(160))
+        );
+}
+
 #if wxUSE_STD_STRING
 void VarArgTestCase::StdString()
 {