To determine the effects of human immunodeficiency virus (HIV) infection on pregnancy outcomes, we prospectively studied female intravenous drug users in a methadone program in New York City. Of 191 women with HIV status known prior to pregnancy, 17 (24%) of 70 seropositives and 26 (22%) of 121 seronegatives became pregnant during 28 months of follow-up. Including 54 additional women first tested for HIV antibody after becoming pregnant, 125 pregnancies were studied in 97 women (39 seropositive, 58 seronegative). None of the seropositive pregnant women had advanced HIV-related disease at entry, and only one developed symptomatic disease (oral candidiasis) during pregnancy. No differences were observed between groups in the frequency of spontaneous or elective abortion, ectopic pregnancy, preterm delivery, stillbirth, or low-birth-weight births. Among women giving birth to live infants, seropositives were more likely than seronegatives to be hospitalized for bacterial pneumonia during pregnancy and had an increased tendency for breech presentation, although these events were infrequent. There were otherwise no differences between groups in the occurrence of antenatal, intrapartum, or neonatal complications. Results suggest that asymptomatic HIV infection is not associated with a decreased pregnancy rate or an increased risk of adverse pregnancy outcomes in intravenous drug users, and that an acceleration in HIV-disease status during pregnancy is uncommon.