The authors report the results of the first 2 years of follow-up of a prospective cohort study on the mother-to-child transmission of human immunodeficiency virus type 1 (HIV-1) and its determinants which started in November 1988 in Kigali, Rwanda. The study sample consists of 218 newborns of 215 HIV-1 seropositive women matched to 218 newborns of 216 HIV-1 seronegative women of the same age and parity. They were followed every 2 weeks during the first 2 years of follow-up. HIV-1 antibodies were detected by enzyme-linked immunoadsorbent assay and Western blot at 3-month intervals. Two methods of calculating the mother-to-child transmission rate were used: method 1 combines the information provided by the persistence of HIV-1 antibodies at 15 months of age in children born to HIV seropositive mothers and the excess mortality in this group compared with the cohort of children born to HIV seronegative mothers; method 2 is a case-by-case evaluation of all the children born to HIV seropositive mothers. A logistic regression model was used to study the determinants of transmission. The probability of survival at 24 months of age was 81% (95% confidence interval (CI) 75-86) in children born to seropositive mothers, compared with 95% (95% CI 92-98) in children born to seronegative mothers (p < 0.001). The mother-to-child transmission rate calculated with method 1 was 25.7% (95% CI 18.8-32.5). With method 2, the medium estimate was 24.7%. In the multivariate analysis, a CD4/CD8 ratio < 0.5 was the only maternal factor statistically associated with an increased risk of mother-to-child transmission of HIV-1 (odds ratio = 2.9, 95% CI 1.2-7.2). The authors' findings present evidence for a higher mother-to-child transmission rate of HIV-1 in children born in Rwanda than in industrialized countries.