codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3c.org/TR/1999/REC-html401-19991224/loose.dtd">
<html xml:lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Test</title>
<script type="text/javascript" language="javascript">
function controlla_quartiere(){
if(document.ricercacase.comune.options[document.ricercacase.comune.selectedIndex].value == "padova")
    document.ricercacase.quartiere.style.visibility = 'visible';
else
    document.ricercacase.quartiere.style.visibility = 'hidden';
}
</script>
</head>
<body>
<form name="ricercacase">
    <select name="comune" onChange="controlla_quartiere()">
        <option value="abano terme">Abano Terme</option>
        <option value="padova">Padova</option>
        <option value="saonara">Saonara</option>
        <option value="mestrino">Mestrino</option>
    </select>
    <select type="text" name="quartiere" maxlength="30" style="visibility:hidden">
        <option value="nord">Nord</option>
        <option value="est">Est</option>
        <option value="sud">Sud</option>
        <option value="ovest">Ovest</option>
        <option value="centro">Centro</option>
    </select>
</form>
</body>
</html>
Vedi ci&ograve; che ho scritto in rosso, con particolare attenzione a:
codice:
if(document.ricercacase.comune.options[document.ricercacase.comune.selectedIndex].value == "padova")
Il test sull'uguaglianza in javascript, cos&igrave; come in java ed in C si scrive == e non = (alla Basic!!!)

HTH
Zappa