Query Posts that are in 2 different categories

I want to use the query posts function in my sidebar.php to display the most recent 5 posts that appear only in both category id 3 and 7. I have the following but it shows all posts in both categories.

<?php query_posts(‘cat=3&7&showposts=5’); ?>

• <h2>Recent Posts</h2>

<?php while (have_posts()) : the_post(); ?>
• ‘><?php the_title(); ?>

<?php endwhile; ?>

How do you query “if in category 3 and 7” show posts in the case above?