diff -Naur HTML-FillInForm-2.00/lib/HTML/FillInForm.pm HTML-FillInForm-2.00.new/lib/HTML/FillInForm.pm
--- HTML-FillInForm-2.00/lib/HTML/FillInForm.pm	2007-09-11 17:20:01.000000000 -0700
+++ HTML-FillInForm-2.00.new/lib/HTML/FillInForm.pm	2007-11-30 16:36:19.000000000 -0800
@@ -42,6 +42,8 @@
         fill_password  =>  1,
         ignore_fields  =>  1,
         disable_fields =>  1,
+        invalid_fields =>  1,
+        invalid_class  =>  1,
     }
 }
 
@@ -128,6 +130,11 @@
     ? @{ $option{disable_fields} } : $option{disable_fields} if exists( $option{disable_fields} );
   $self->{disable_fields} = \%disable_fields;
 
+  my %invalid_fields;
+  %invalid_fields = map { $_ => 1 } ( ref $option{'invalid_fields'} eq 'ARRAY' )
+    ? @{ $option{invalid_fields} } : $option{invalid_fields} if exists( $option{invalid_fields} );
+  $self->{invalid_fields} = \%invalid_fields;
+
   if (my $fdat = $option{fdat}){
     # Copy the structure to prevent side-effects.
     my %copy;
@@ -158,6 +165,12 @@
     $self->{'target'} = $target;
   }
 
+  if (my $invalid_class = $option{invalid_class}){
+    $self->{'invalid_class'} = $invalid_class;
+  } else {
+    $self->{'invalid_class'} = 'invalid';
+  }
+
   if (defined($option{fill_password})){
     $self->{fill_password} = $option{fill_password};
   } else {
@@ -220,6 +233,23 @@
     exists $self->{disable_fields}{ $attr->{'name'} } and
     $self->{disable_fields}{ $attr->{'name'} } and
     not ( exists $attr->{disable} and $attr->{disable} );
+
+  # Check if we need to invalidate this field
+  my $invalidating = 0;
+  if (exists $attr->{name} and
+      exists $self->{invalid_fields}{ $attr->{name} } and
+      $self->{invalid_fields}{ $attr->{name} }) {
+      $invalidating = 1;
+      if (exists $attr->{class} and length $attr->{class}) {
+          # don't add the class if it's already there
+          unless ($attr->{class} =~ /\b\Q$self->{invalid_class}\E\b/) {
+              $attr->{class} .= " $self->{invalid_class}";
+          }
+      } else {
+          $attr->{class} = $self->{invalid_class};
+      }
+  }
+
   if ($tagname eq 'input'){
     my $value = exists $attr->{'name'} ? $self->_get_param($attr->{'name'}) : undef;
     # force hidden fields to have a value
@@ -336,7 +366,18 @@
       $self->{selectMultiple} = 0;
       $self->{selectSelected} = 0; # helper var to remember if an option was already selected in the current select tag
     }
-    $self->{output} .= $origtext;
+
+    # need to re-output the <select> if we're marking it invalid
+    # (doesn't disable need this too?)
+    if ($invalidating) {
+        $self->{output} .= "<$tagname";
+        while (my ($key, $value) = each %$attr) {
+            $self->{output} .= sprintf qq( %s="%s"), $key, $value;
+        }
+        $self->{output} .= ">";
+    } else {
+        $self->{output} .= $origtext;
+    }
   } else {
     $self->{output} .= $origtext;
   }
@@ -530,6 +571,18 @@
 
     disable_fields => [ 'uid', 'gid' ]
 
+=head3 invalid_fields => []
+
+To mark fields as being invalid (CSS class set to "invalid" or
+whatever you set invalid_class to):
+
+    invalid_fields => [ 'uid', 'gid' ]
+
+=head3 invalid_class => "invalid"
+
+The CSS class which will be used to mark fields invalid.  Defaults to
+"invalid".
+
 =head2 File Upload fields
 
 File upload fields cannot be supported directly. Workarounds include asking the
diff -Naur HTML-FillInForm-2.00/t/23_invalid_fields.t HTML-FillInForm-2.00.new/t/23_invalid_fields.t
--- HTML-FillInForm-2.00/t/23_invalid_fields.t	1969-12-31 16:00:00.000000000 -0800
+++ HTML-FillInForm-2.00.new/t/23_invalid_fields.t	2007-11-30 16:31:13.000000000 -0800
@@ -0,0 +1,48 @@
+#!/usr/local/bin/perl
+
+# contributed by Trevor Schellhorn
+
+use strict;
+use warnings FATAL => 'all';
+
+use Test::More tests => 10;
+
+use_ok('HTML::FillInForm');
+
+my $html = qq[
+<form>
+<input type="text" name="one" value="all wrong">
+<input type="text" name="two" class="existing" value="worse">
+<input type="text" name="three" class="invalid" value="already bad">
+<select name="four">
+  <option value="1">Foo</option>
+  <option value="2">Boo</option>
+</select>
+</form>
+];
+
+my $result =
+  HTML::FillInForm->new->fill(scalarref      => \$html,
+                              fdat           => {two => "new val 2"},
+                              invalid_fields => ['one']);
+
+like($result, qr/<input[^>]+name="one"[^>]+class="invalid"/);
+unlike($result, qr/<input[^>]+name="two"[^>]+class="invalid"/);
+
+$result = HTML::FillInForm->new->fill(scalarref      => \$html,
+                                      fdat           => {two => "new val 2"},
+                                      invalid_fields => ['one', 'two', 'three', 'four']);
+
+like($result, qr/<input[^>]+name="one"[^>]+class="invalid"/);
+like($result, qr/<input[^>]+name="two"[^>]+class="existing invalid"/);
+like($result, qr/<input[^>]+name="three"[^>]+class="invalid"/);
+like($result, qr/<select[^>]+name="four"[^>]+class="invalid"/);
+
+$result = HTML::FillInForm->new->fill(scalarref      => \$html,
+                                      fdat           => {two => "new val 2"},
+                                      invalid_fields => ['one', 'three'],
+                                      invalid_class  => "funky");
+
+like($result, qr/<input[^>]+name="one"[^>]+class="funky"/);
+like($result, qr/<input[^>]+name="two"[^>]+class="existing"/);
+like($result, qr/<input[^>]+name="three"[^>]+class="invalid funky"/);