tabless full height column

I'd like a column that flows the full height of a page, even if you
scroll down.

If I did this:

<div style="width: 100px;height=100%;background-color: red">
side bar
</div>
<div style="float: left; background-color: #fff">
main content, enough to have vertical scroll bars on the browser
</div>

I'd have problems after I scrolled down.

What do I need to do? Seems like I should know this, but... I'm
drawing a blank.

Jeff