We analyzed the association of herpes simplex virus (HSV) infection and syphilis, the two most common causes of genital ulceration in homosexual men, with human immunodeficiency virus (HIV) infection in 200 men enrolled between 1983 and 1986 into a study evaluating the microbial causes of acute proctitis. Infection with HIV was independently associated with a history of syphilis, serologic evidence of syphilis, a history of HSV infection, and antibody to HSV