Anal cancer is associated with a virus (human papillomavirus, or HPV) that causes genital warts. People get the virus from unprotected sexual intercourse. The virus can produce abnormal cells (precancerous and cancerous) and is the same virus that increases the risk for cervical cancer in women. People who have reduced immunity, such as patients with HIV infection, are at increased risk for developing cancers that are related to HPV infection. Men who have receptive anal intercourse have a high incidence of anal cancer. Both HPV infection and anal cancer are more common in HIV-positive men who have sex with men than in HIV-negative homosexual men. However, little is known about HPV and anal lesions in HIV-positive men who have no history of receptive anal intercourse.