The incidence of male-to-female transmission of HIV infection was studied in a population of 198 sexual partners of hemophiliacs who tested HIV positive since 1984. The follow-up observation period was 1987-1992. Transmission occurred in 20 (10%) cases. The analysis of risk factors for transmission was performed in a subgroup of 57 hemophiliacs with seronegative sexual partners as compared to eight transmitters. Transmitters showed a significantly more advanced immune depletion at enrollment as well as at the end of the observation period. Furthermore, transmitters had a more advanced disease at the end of the study (75% vs. 29% CDC IV; p < 0.01). Also virus cultures were more frequently positive in the transmitters than in the non-transmitters (71% vs. 42%). Regular sexual counseling was offered to all couples. After 1987, no new seroconversions were detected. However, two seroconversions in female partners of hemophiliacs outside the initial study population were observed. Both transmissions occurred during a period of severe clinical and immunological deterioration. This study shows that sexual partners of HIV-infected hemophiliacs with more advanced disease are at higher risk of infection with HIV. The frequency of male-to-female transmission of HIV in long-term monogamous sexual relationships practicing safer sex is low. Overall, disease awareness and counseling for safer sex seem to be effective in reducing transmission rates.