This discussion of the pathogenesis of infant human immunodeficiency virus (HIV) type 1 infection emphasizes features unique to pediatric disease and provides general insights into HIV illness in all populations. Therapies designed to interrupt transmission of HIV can be most efficiently addressed through studies of maternal-infant transmission. The rapid progression of disease in children constantly emphasizes the urgent need for continued progress in treatment of HIV infection and presents a unique opportunity optimally to assess effects of early intervention.