Usar onChange em tag

Estou tentando usar o resultado do primeiro combobox para fazer a consulta da segunda combobox.

Alguem pode me ajudar sou novato.

<form name="frmConsulta" action="Admin_Boletim.asp" method="post">
<select name="cboSetor" class="Arial9_CZE" onChange="document.getElementById('frmConsulta').submit()">
 <option value="" selected>-escolha-</option>
 <%
 varSQL="SELECT * FROM tb_Boletim_Setor ORDER BY Setor"
 Set rsC = Server.CreateObject("ADODB.Recordset")
 rsC.open varSQL,varConINFO,3,1
 If NOT rsC.EOF OR NOT rsC.BOF Then
 do while not rsc.eof
 %>
 <option value="<%=rsC.Fields.Item("NumSetor").Value%>"<% if CLng(varNumSetor_C)=CLng(rsC.Fields.Item("NumSetor").Value) then %> selected<% end if %>><%=rsC.Fields.Item("Setor").Value%></option>
 <%
 rsc.movenext
 loop
 end if
 rsc.close
 set rsc=nothing
 varSQL=""
 %>
 </select></td>
 </tr>
 <tr bgcolor="#F5F8FF">
 <td><img src="../Imagens/Blank.GIF" width="10" height="5"></td>
 <td class="Arial9_CZE_B" align="right">Assunto: </td>
 <td colspan="2"><select name="cboAssunto" class="Arial9_CZE">
 <option value="" selected>-escolha-</option>
 <%
 varSQL="SELECT * " &_
 "FROM tb_Boletim_Assunto " &_
 "WHERE (NumSetor = "& request.form("cboSetor") &") " &_
 "ORDER BY Assunto "
 Set rsC = Server.CreateObject("ADODB.Recordset")
 rsC.open varSQL,varConINFO,3,1
 If NOT rsC.EOF OR NOT rsC.BOF Then
 do while not rsc.eof
 %>
 <option value="<%=rsC.Fields.Item("NumAss").Value%>"<% if CLng(varNumAss_C)=CLng(rsC.Fields.Item("NumAss").Value) then %> selected<% end if %>><%=rsC.Fields.Item("Assunto").Value%></option>
 <%
 rsc.movenext
 loop
 end if
 rsc.close
 set rsc=nothing
 varSQL=""
 %>
 </select>