jstl drop down B depends on drop down A

How do i do to populate a second Drop down which depends on the value i selected from a previous drop down list?

So far the way i populate a single drop down list is like this.. Before loading the page i put in my httpSession a List with the data i will populate.. and when i select i put the value of that option into a hidden input Text Field..

<select class="combo" name="selectDependencias"onChange="this.form.inputDependencia.value = this.value;">

<option selected></option>

<c:forEach items='${sessionScope.dependencias}' var='EDependencia'>

<option value="<c:out value='${EDependencia.cod_dependencia}'/>">

<c:out value='${EDependencia.nom_dependencia}'/>

</option>

</c:forEach>

</select>

[1006 byte] By [pompeighuIIa] at [2007-11-27 10:05:31]