The risk of acquiring a sexually transmitted disease as a result of rape is not known, in part because it is difficult to ascertain whether infections were present before the assault or acquired during it. To investigate this question, we examined female victims of rape within 72 hours of the assault and again at least one week after the assault. Of the 204 girls and women initially examined within 72 hours of the rape, 88 (43 percent) were found to have at least one sexually transmitted disease. These diseases included infections caused by Neisseria gonorrhoeae (6 percent of those tested), cytomegalovirus (8 percent), Chlamydia trachomatis (10 percent), Trichomonas vaginalis (15 percent), herpes simplex virus (2 percent), Treponema pallidum (1 percent), and the human immunodeficiency virus type 1 (HIV-1; 1 percent) and bacterial vaginosis (34 percent). Among the 109 patients (53 percent) who returned for at least one follow-up visit (excluding those who were found to be infected at the first visit or who were treated prophylactically), the incidence of new disease was as follows: gonorrhea, 4 percent (3 of 71); chlamydial infection, 2 percent (1 of 65); trichomoniasis, 12 percent (10 of 81); and bacterial vaginosis, 19 percent (15 of 77). There were no new infections with herpes simplex virus, cytomegalovirus, Trep. pallidum, or HIV-1, but follow-up serologic testing was performed in only 26 percent of the patients. On the basis of our assumptions that most venereal infections present within 72 hours of a rape were preexisting and that new infections identified 1 to 20 weeks later were acquired during the assault, we conclude that the prevalence of preexisting sexually transmitted diseases is high in victims of rape and that they have a lower but substantial additional risk of acquiring such diseases as a result of the assault.