According to the New York Times,

Circumcision appears to reduce a man’s risk of contracting AIDS from heterosexual sex by half, United States government health officials said yesterday ….

The National Institutes of Health has its theories about why this is:

Uncircumcised men are thought to be more susceptible because the underside of the foreskin is rich in Langerhans cells, sentinel cells of the immune system, which attach easily to the human immunodeficiency virus, which causes AIDS. The foreskin also often suffers small tears during intercourse.