Forse ti aiuterà....
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
</head>
<body>
<script language="JavaScript" type="text/javascript">
<!--
var citta = new Array();
citta[0] = new Array();
citta[0][0] = "Colmar";
citta[0][1] = "Kaisersberg";
citta[0][2] = "Kunheim";
citta[0][3] = "Strasbourg";
citta[1] = new Array();
citta[1][0] = "Bègles";
citta[1][1] = "Bordeaux";
citta[1][2] = "La Rochelle";
citta[2] = new Array();
citta[2][0] = "Aubervilliers";
citta[2][1] = "Malakoff";
citta[2][2] = "Montrouge";
citta[2][3] = "Paris";
citta[3] = new Array();
citta[3][0] = "Aix";
citta[3][1] = "Marseille";
citta[3][2] = "Nice";
citta[3][3] = "Toulon";
citta[4] = new Array();
citta[4][0] = "Bourg en Bresse";
citta[4][1] = "Lyon";
citta[4][2] = "Saint Etienne";
function change(indice) {
var cittaSel = document.getElementById('citta');
for (i=cittaSel.options.length-1;i>=0;i--) {
cittaSel.options[i] = null;
} // for (i=cittaSel.options.length-1;i>=0;i--)
if (indice != "-") {
for (i=0;i<=citta[indice].length-1;i++) {
document.getElementById('citta').options[i] = new Option(citta[indice][i], i);
} // for (i=0;i<=citta[indice].length-1;i++)
} // if (indice != "-")
} // function change(indice)
//-->
</script>
<table>
<tr>
<td>
Regione
</td>
<td>
<select id="provincia" onchange="change(this.value);">
<option value="-">Seleziona una regione</option>
<option value="0">Alsace</option>
<option value="1">Aquitaine</option>
<option value="2">Ile de France</option>
<option value="3">PACA</option>
<option value="4">Rhône-Alpes</option>
</select>
</td>
</tr>
<tr>
<td>
Città
</td>
<td>
<select id="citta">
</select>
</td>
</tr>
</table>
</body>
</html>