Category descriptions next to posts

I’ve been searching loads and have not found a fix that will work specifically for what I’m trying to do. When someone clicks on a post to read the whole thing, I want the category description to appear at the top of the sidebar next to the post.

Something like this <p><?php echo category_description(3); ?></p> is a static, sticky description get. What i want to do is display the description that matches the category for whatever post they are looking at. What am I doing wrong or what do I need to do?

Much thanks in advance.

Oh, this is an MU installation, though since it’s just for a specific blog in it, I don’t think that matters