Slette i MySQL

Hvorfor virker dette script ikke? Får ingen fejl, men der slettes ikke noget i databasen.

<?php
if ($submit) {
    mysql_query("DELETE FROM downloads WHERE id='$ID'") or die (mysql_error());
    echo "<script> alert('slettet!'); </script>";
}
?>

<form name="form" action="<?=$PHP_SELF;?>" method="post" OnSubmit="return confirm('Er du sikker på, at du vil slette?');">
<table width="400" border="1" cellspacing="0" cellpadding="2">
<?php
// Connect to db
include($_SERVER['DOCUMENT_ROOT']."/sql.php");
$query = mysql_query("SELECT * FROM downloads ORDER BY ID ASC");
$antal = mysql_num_rows($query);
while($row = mysql_fetch_array($query)) {
?>
    <tr>
        <td><input type="radio" name="id" value="<?=$row['ID'];?>">
        <td><?=$row['titel'];?></td>
    </tr>
<?php
}
?>
</table>
<br>
<input type="submit" name="submit" value="  slet  ">
</form>