codice:
<script>
if (document.getElementById('professione').value=="Area Commerciale")
{
    document.getElementById('areanascosta').style.display="block";
}
else
{
    document.getElementById('areanascosta').style.display="none";
}
</script>





<div class="form_item">
  <div class="form_element cf_textbox">
    <label class="cf_label" style="width: 150px;">Nome:</label>
    <input class="cf_inputbox required" maxlength="150" size="30" title="inserisci il tuo nome" id="nome" name="nome" type="text" />
  
  </div>
  <div class="cfclear"></div>
</div>

<div class="form_item">
  <div class="form_element cf_textbox">
    <label class="cf_label" style="width: 150px;">Cognome:</label>
    <input class="cf_inputbox required" maxlength="150" size="30" title="inserisci il tuo cognome" id="cognome" name="cognome" type="text" />
  
  </div>
  <div class="cfclear"></div>
</div>

<div class="form_item">
  <div class="form_element cf_textbox">
    <label class="cf_label" style="width: 150px;">E mail:</label>
    <input class="cf_inputbox required" maxlength="150" size="30" title="inserici la tua E-mail" id="mail" name="mail" type="text" />
  
  </div>
  <div class="cfclear"></div>
</div>

<div class="form_item">
  <div class="form_element cf_dropdown">
    <label class="cf_label" style="width: 150px;">Profilo professionale:</label>
    <select class="cf_inputbox validate-selection" id="professione" size="1" title="inserisci una professione"  name="profilo" onChange="mostra()">
    <option value="">Scegli la tua professione</option>
      <option value="Programmatore">Programmatore</option>
<option value="Net Content Manager">Net Content Manager</option>
<option value="Web Designer">Web Designer</option>
<option value="Area Commerciale">Area Commerciale</option>

    </select>
    
  </div>
  <div class="cfclear"></div>
</div>
<div class="maremma" style="display:none" id="areanascosta">
<div class="form_item">
  <div class="form_element cf_text"> <span class="cf_text">solo se si è scelto Area commerciale:</span> </div>
  <div class="cfclear"></div>
</div>

<div class="form_item">
  <div class="form_element cf_dropdown">
    <label class="cf_label" style="width: 150px;">Area Commerciale</label>
    <select class="cf_inputbox" id="area" size="1" title=""  name="area">
    <option value="">Scegli la tua area commerciale</option>
      <option value="Professionisti">Professionisti</option>
<option value="Commercianti">Commercianti</option>
<option value="Piccola impresa">Piccola impresa</option>
<option value="Media impresa">Media impresa</option>
<option value="Grandi aziende">Grandi aziende</option>
<option value="Gruppi imprenditoriali">Gruppi imprenditoriali</option>

    </select>
    
  </div>
</div>
  <div class="cfclear"></div>
</div>

<div class="form_item">
  <div class="form_element cf_textbox">
    <label class="cf_label" style="width: 150px;">Url profilo su LinkedIn:</label>
    <input class="cf_inputbox" maxlength="150" size="30" title="" id="linkedin" name="linkedIn" type="text" />
  
  </div>
  <div class="cfclear"></div>
</div>

<div class="form_item">
  <div class="form_element cf_textarea">
    <label class="cf_label" style="width: 150px;">Messaggio:</label>
    <textarea class="cf_inputbox" rows="3" id="messaggio" title="" cols="30" name="messaggio"></textarea>
    
  </div>
  <div class="cfclear"></div>
</div>

<div class="form_item">
  <div class="form_element cf_fileupload">
    <label class="cf_label" style="width: 150px;">Invia C.V.</label>
    <input class="cf_fileinput cf_inputbox" title="" size="20" id="cv" name="cv" type="file" />
    
  </div>
  <div class="cfclear"></div>
</div>

<div class="form_item">
  <div class="form_element cf_button">
    <input value="Invia" name="button_9" type="submit" /><input type="reset" value="Reset"/>
  </div>
  <div class="cfclear"></div>
</div>