A cross-sectional study of stable monogamous couples, recruited from 16 clinical and surveillance centers in Italy between 1987 and 1992, was carried out to investigate the risk factors of woman-to-man sexual transmission of human immunodeficiency virus (HIV). The male partners of all HIV-infected women attending the centers were invited to participate in the study. Of the 275 male partners who were tested for HIV and interviewed with use of a structured questionnaire, 51 were excluded because they had other possible risk factor for HIV infection, no established risk factor was found in the index case, or they had stopped engaging in sexual intercourse. Fourteen of the 224 men (6.3%) were seropositive for HIV. At logistic regression, the highest risks of transmission were for men practicing peno-anal intercourse [odds ratio (OR), 4.6; 95% confidence interval (CI), 1.0-22.2] and for men whose partner had acquired immune deficiency syndrome (AIDS) or a CD4+ lymphocyte count of < or = 400/mm3. No seropositive men were observed among those who were aware of the woman's HIV seropositivity since the beginning of the relationship or were partners of a zidovudine-treated woman. The results suggest that the risk factors described in man-to-woman and man-to-man HIV sexual transmission also operate in woman-to-man transmission.