query_posts questions?

I am trying to populate a section on my websites home page, that grabs posts that are part of two categories.

To be more specific, i want to only show a post on the home page, if it is part of Category 3 AND 7, not 3 OR 7. Which appears to be what is happening. Here is the code I am using:

query_posts(“cat=5&cat=3&showposts=1”);

The way I thought this would work is it would look and see if it is in cat 5, and then if it is also in cat 3 as well. Any help here?