PIP: To evaluate the epidemiologic significance of breastfeeding to the transmission of human immunodeficiency virus (HIV) in a country with a high prevalence of HIV infection, the 1720 seronegative women who delivered at the University Teaching Hospital in Lusaka, Zambia, in a 3- month period in 1987 were enrolled in a longitudinal study. Only 634 (37%) of these women returned for testing at the 1-year follow-up point. Of these, 19 (3%) had become seropositive. The infection was asymptomatic in all 19 women at the time of the 1-year follow-up; however, 5 of these women soon developed generalized persistent adenopathy and 3 had spontaneous abortions during the year in which seroconversion occurred. 30 of the spouses of the women in the study sample were HIV-positive; the relative risk of seroconversion was 3.84 in women with HIV-infected spouses compared to those with HIV-negative spouses. Other significant risk factors for HIV seroconversion included: history of genital ulceration after delivery (relative risk, 15.51), use of a cloth to remove vaginal secretions during intercourse (dry sex) (relative risk, 37.95), and blood transfusion (relative risk, 10.89). 3 infants born to these 19 women also seroconverted; 2 years after seroconversion, only 1 of the 3 infected children was symptomatic (persistent, generalized lymphadenopathy). Other sources of HIV infection 9e.g., scarification, blood transfusions, use of contaminated needles during immunization) aside from breastfeeding were not recorded in these 3 infants. Although there is a high prevalence of HIV infection in Zambia, the health benefits of breastfeeding (in terms of the prevention of mortality from diarrheal disease) still outweigh the small risk of HIV transmission.