A las buenas,
a ver si te sirve este ejemplo:
Código PHP:
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="javascript">
function cambiar(cual,valor)
{
if(valor==0){cual.pepe.style.visibility='hidden'}
else{
cual.pepe.style.visibility='visible'}
}
</script>
</head>
<body bgcolor="#FFFFFF" text="#000000">
<form name="form1" method="post" action="">
<select name="opciones" onChange="cambiar(this.form,this.selectedIndex)">
<option selected>Seleccionar</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
<input type="text" name="pepe" style="visibility:hidden">
</form>
</body>
</html>
