Ho apportato le modifiche che mi hai consigliato, e mi sembra che questo script faccia prorpio al caso mio ma non so perchè non funzia...ti passo il codice:

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<html>
<head>
<title>Titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

<script language="JavaScript" type="text/JavaScript">

function Cambia(valore) {
modulo=document.form;
switch(valore) {
case 1:
modulo.B.options[0]=new Option('testo1a','valore1a');
modulo.B.options[1]=new Option('testo1b','valore1b');
modulo.B.options[2]=new Option('testo1c','valore1c');
break;
case 4:
modulo.B.options[0]=new Option('testo2a','valore2a');
modulo.B.options[1]=new Option('testo2b','valore2b');
modulo.B.options[2]=new Option('testo2c','valore2c');
break;
case 5:
modulo.B.options[0]=new Option('testo3a','valore3a');
modulo.B.options[1]=new Option('testo3b','valore3b');
modulo.B.options[2]=new Option('testo3c','valore3c');
break;
}
}

</script>
</head>

<body>

<form name="form" method="get" action="top_dx.php" target="rightFrame">
<table width="337">
<tr>
<td>Citt&agrave;:
<select name="A" onChange="Cambia(this[this.selectedIndex].value);">





<option value="%">Tutte le città</option>
<option value="1">Rovigo</option>
<option value="4">Milano</option>
<option value="5">Milano</option>
<option value="6">Torino</option>
<option value="7">Roma</option>
<option value="8">Veneziaa</option>
<option value="10">Berlino</option>
<option value="11">amsterdam</option>
</select>
</td>
<td>
<select name="B">
</select>
</td>

<td> </td>
</tr>
</table>
</form>
</body>
</html>

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