The role of the human immunodeficiency virus (HIV) and other sexually transmitted diseases (STDs) as risk factor for spontaneous abortion was investigated in a case-control study in Nairobi, Kenya. Cases (n = 195) were women admitted with clinical signs and symptoms of spontaneous abortion, before 20 weeks of gestation. Patients with induced or clinically septic abortion were excluded. Controls were unselected pregnant women in their second or third trimester (n = 195). Spontaneous abortion was independently associated with maternal HIV-1 antibody (14.4% versus 6.2%; adjusted odds ratio, 2.3; 95% confidence interval, 1.1-4.8), with maternal syphilis seroreactivity (6.8% vs 2.1%, adjusted odds ratio, 4.3; 95% confidence interval 1.2-15.6), and with vaginal colonization with group B streptococci (15.6% vs 5.2%; adjusted odds ratio, 3.2; 95% confidence interval, 1.5-6.7). No significant association was found between maternal infections with gonococci and genital mycoplasmas, and spontaneous abortion.