... qui:

codice:
<html>
<head>
<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; 
} 
} 

function Cancella(myform) {
 var elementi=myform.contenitore.options;
 var valori=new Array();
 var testi=new Array();
 for (i=0; i<elementi.length; i++)
  if (elementi[i].selected) break;
 k=0;
 for (j=0; j<elementi.length; j++) {
  if (i!=j) {
   valori[k]=elementi[j].value;
   testi[k]=elementi[j].text;
   k++
  }
 }
 for (i=0; i<valori.length; i++) {
  myform.contenitore.options[i]=new Option(valori[i], testi[i]);
 }
 myform.contenitore.options.length=valori.length;
}

function Sposta(cosa) {
 var contenitore=document.form.contenitore;
 var quanti=contenitore.options.length;
 var trovato=false;
 for (i=0; i<quanti; i++) if (contenitore[i].value==cosa) trovato=true;
 if (cosa!="Nulla" && !trovato)
 contenitore.options[quanti]=new Option(cosa,cosa);
}

</script> 
</head>
<body> 

<form name="form" method="get" action="top_dx.php" target="rightFrame"> 
<table width="337"> 
<tr> 
<td>Città: 
<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" onChange="Sposta(this[this.selectedIndex].value);"> 
</select> 
</td> 

<td> </td> 
</tr> 
</table> 

  <select name="contenitore" size="5">
  </select>
  <input type="button" name="Submit" value="Cancella" onClick="Cancella(this.form);">
</form> 
</body> 
</html>
Ciao !