Thirty-four human immunodeficiency virus type 1 (HIV-1)-infected mother-child pairs were studied from delivery, and the association between velocity of disease progression in mothers and age at AIDS appearance in their children was analyzed. Four maternal disease stages were defined, and the velocity of HIV-1 progression was calculated. Maternal clinical conditions at delivery and intervals before starting zidovudine in women and children were included in the analysis. The relative risk of the child's developing AIDS for mother's disease progression of 1 stage/year was 3.004 (95% confidence interval, 1.37-6.58, P = .006) in stepwise analysis. A maternal CD4+ cell count < or = 400/mm3 appeared to be associated with a rapid disease course in both women and children. Thus, the faster HIV-1 progressed in the mother, the quicker AIDS developed in her baby. This suggests that HIV-1 strains with comparable virulence develop in mother-child pairs, perhaps because of their similar genetically driven immune responses.