making iframe take up 100% of height

Hi,
I am trying to get my iframe to occupy the entire height of the
table cell in which it is located. Sadly, this table cell may have
variable height and the height=100% attribute doesn't seem to wokr on
an iframe in either PC IE or Firefox. This is the code I'm using right
now that doesn't work on any browser ...

<tr>
<td><iframe style="background-color:#ffffff;
border-style:none;
border-width:0px;" width="100%" height="100%" src="features.php"
scrolling="auto"></iframe></td>
</tr>

Does anyone have any suggestions?

THanks, - Dave