faccio così:

codice:
<script type="text/javascript">
<!--//
function controlla(form) {
Numero = document.DatiFam.txtNum.value;
alert(Numero);
}
//-->
</script>

<%
c=1
do while not rsFam.eof
	if c mod 2 = 0 then
		response.write "<tr class = """ & "rp" & """" & ">"
	else
		response.write "<tr class = """ & "rd" & """" & ">"
	end if%>
    <td width="30%"><%=rsfam(0)%></td>
    <td width="20%"><%=rsfam(1)%><imput type="hidden" name="txtCFFam" value="<%=rsfam(1)%>"></td>
    <td width="5%"><input class="regR" type="text" name="txtNum" size="5%" onblur='controlla(this.form)' value="<%=cint(rsFam(5)/rsFam(4))%>" ></td>     <td width="15%"><%=rsfam(3)%><imput type="hidden" name="txtCau" value="<%=rsfam(2)%>"></td>
    <td width="10%" align="center"><%=rsfam(4)%></td>
    <td width="10%"><input class="regR" type="text" name="txtImporto_<%=rsFam(6)%>" size="10%" value="<%=rsfam(5)%>"></td>
  <%
	rsFam.movenext
 	c=c+1
loop
%>
ma sull'onblur mi fa l'alert con "undefined"