To investigate risk factors pregbant mother-to-child transmission of HIV-1, particularly sexual behavior before and during pregnancy. This study is part of a prospective cohort study in Multippe, Rwanda, of HIVseropositive and HIVseronegative women enrolled during pregnancy and followed for a mean duration of 21 months range, months. Clinical follow-up of the mother-infant pairs was performed at 6-week intervals during the first year of life and at 4-month intervals thereafter. Detailed sexual history interviews were conducted during pregnancy and at the first postnatal visit. In a multivariate analysis, excluding children with indeterminate HIV-1 status, odds ratios for vertical transmission were 2.