Problema risolto.
parte del codice l'ho trovato nel sito di html.
bisogna associare ai valori la corrispondente pagina htm (es. 64 ---> 64.htm)
ecco il codice:


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Documento senza titolo</title>
<SCRIPT type=text/javascript>

var regiondb = new Object()
regiondb["africa"] = [{value:"102", text:"Cairo"},
{value:"88", text:"Lagos"},
{value:"80", text:"Nairobi"},
{value:"55", text:"Pretoria"}];
regiondb["asia"] = [{value:"30", text:"Ankara"},
{value:"21", text:"Bangkok"},
{value:"49", text:"Pechino"},
{value:"76", text:"New Delhi"},
{value:"14", text:"Tokyo"}];
regiondb["australia"] = [{value:"64", text:"Suva"},
{value:"12", text:"Sydney"}];
regiondb["europa"] = [{value:"11", text:"Atene"},
{value:"35", text:"Francoforte"},
{value:"3", text:"Londra"},
{value:"15", text:"Madrid"},
{value:"1", text:"Parigi"},
{value:"10", text:"Roma"},
{value:"6", text:"Stoccolma"},
{value:"97", text:"San Pietroburgo"}];
regiondb["noamer"] = [{value:"73", text:"Dallas"},
{value:"71", text:"Los Angeles"},
{value:"5", text:"New York"},
{value:"37", text:"Toronto"}];
regiondb["suamer"] = [{value:"65", text:"Buenos Aires"},
{value:"31", text:"Caracas"},
{value:"66", text:"Rio di Janeiro"}];

function setCities(chooser) {
var newElem;
var where = (navigator.appName == "Microsoft Internet Explorer") ? -1 : null;
var cityChooser = chooser.form.elements["città"];
while (cityChooser.options.length) {
cityChooser.remove(0);
}
var choice = chooser.options[chooser.selectedIndex].value;
var db = regiondb[choice];
newElem = document.createElement("option");
newElem.text = "Seleziona una città:";
newElem.value = ".htm";
cityChooser.add(newElem, where);
if (choice != "") {
for (var i = 0; i < db.length; i++) {
newElem = document.createElement("option");
newElem.text = db.text;
newElem.value = db[i].value;
cityChooser.add(newElem, where);
}
}
}

function MM_jumpMenu(targ,selObj,restore){ //v3.0
eval(targ+".location='"+selObj.options[selObj.selectedIndex].value+".htm'");
if (restore) selObj.selectedIndex=0;
}
//-->
</SCRIPT>
</head>
<body>


</p>
<FORM name=form>Invia la richiesta a: <SELECT onchange=setCities(this)
name=continente> <OPTION value="" selected>Seleziona il porto di partenza</OPTION>
<OPTION value=africa>Africa</OPTION> <OPTION value=asia>Asia</OPTION> <OPTION
value=australia>Australia/Oceania</OPTION> <OPTION
value=europa>Europa</OPTION> <OPTION value=noamer>Nord America</OPTION>
<OPTION value=suamer>Sud America</OPTION></SELECT>
<SELECT name=città onChange="MM_jumpMenu('parent',this,0)">
<OPTION value="" selected>Seleziona il porto di arrivo</OPTION></SELECT>
</FORM>
</body>
</html>