codice:
  <html>

    <head>
      <script type="text/javascript" language="JavaScript">
      <!--


        var region = new Array( );

        // Valori che devi impostare te...
        // region["nome_regione"] = new Array("citta_1", "citta_2", "citta_n");

        region["Roma"]    = new Array("Fiumicino", "Pisa", "Caserta");
        region["Milano"]  = new Array("Napoli", "Siracusa");
        region["Bari"]    = new Array("Albachiara", "Roma", "Brebei");
        region["Pescara"] = new Array("Amsterdam", "Udine", "Pordenone", "Massa");


        function populateRegion( __hForm )
        {
          var _hReg = __hForm.elements["region"];
          _hReg.options.length = 0;

          for ( var i in region )
            _hReg.options[_hReg.options.length] = new Option( i, i, false, false );

          populateCity( __hForm );
        }

        function populateCity( __hForm )
        {
          var _hReg     = __hForm.elements["region"];
          var _hCity    = __hForm.elements["city"];
          var _szCities = region[ _hReg.options[_hReg.selectedIndex].value ];

          _hCity.options.length = 0;

          for ( var i=0; i<_szCities.length; i++  )
            _hCity.options[_hCity.options.length] = new Option( _szCities[i], _szCities[i], false, false );
        }


      //-->
      </script>
    </head>

    <body onload="populateRegion( document.forms[0] );">

      <form>
        Provincia:
        <select name="region" onchange="populateCity( this.form );"></select>



        Citta':
        <select name="city"></select>


      </form>

    </body>

  </html>