Cases of mother-to-child transmission (MTCT) of human immunodeficiency virus type 1 (HIV-1) in a prospective cohort study in Latin America and the Caribbean were analyzed. Eight of 820 eligible infants became infected [transmission rate, 0.98% (95% CI = 0.45-1.96%)]. Five cases (62%) represented missed opportunities for prevention of MTCT of HIV-1, suggesting the need for ongoing training and education of clinicians regarding prevention of MTCT of HIV-1.