codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Pagina senza titolo</title>

    <script language="javascript" type="text/javascript">
// <!CDATA[


function dropdown_onchange(v) 
{
    var index = v.selectedIndex;
    var t = document.getElementById("marchio_sigla");
    if(index > 0) t.style.display = ""; else t.style.display = "none";
}

// ]]>
    </script>

</head>
<body>
    <select name="dropdown" onchange="return dropdown_onchange(this)">
        <option value="variante_1">non visualizza</option>
        <option value="variante_2">visualizza</option>
        <option value="variante_3">visualizza</option>
    </select>
    <input id="marchio_sigla" type="text" style="display:none;" />
</body>
</html>