We evaluated risk factors for human immunodeficiency virus (HIV) seropositivity in the wives/female sexual partners of hemophiliacs. One of the 6 seropositive women has developed the acquired immunodeficiency syndrome (AIDS). At least three of the others seroconverted during 1985-86, after more than four years of regular sexual contact with a seropositive hemophiliac. HIV seropositivity was present in at least 5 (50%) of the 10 female partners of hemophiliacs with severe depletion of T-helper cells (less than 100/microliters), including 3 (43%) of the 7 female partners of hemophiliacs with AIDS. All 4 HIV seropositive women with available data had vaginal intercourse without a condom, but risk of seropositivity did not appear to be affected by the frequency of vaginal intercourse or by hysterectomy. Other sexually transmitted diseases, anal intercourse, or vaginal intercourse during menstrual periods were not necessary for HIV infection. These data suggest that heterosexual transmission of HIV can occur during routine vaginal intercourse but that it usually does not occur until the hemophiliac has severe immune deficiency late in the course of HIV infection.