To examine the possible influence of obstetric factors, substance use during pregnancy, and other maternal factors on the relationship between a low maternal CD4+ level and vertical transmission of human immunodeficiency virus type 1 (HIV-1), data were analyzed from the Mothers and Infants Cohort Study, a prospective cohort followed for up to 4 years between 1986 and 1992 in Brooklyn and the Bronx, New York. The overall transmission rate for the cohort was 25.1% (95% confidence interval (CI) = 19.0-31.3). Prenatal CD4+ lymphocyte measurements were available for 162 HIV-seropositive mothers of infants with known infection outcomes. Among mothers who smoked cigarettes after the first trimester, those whose mean prenatal CD4+ level was < 20% had more than a threefold increased risk of transmitting their infection to their infants [relative risk (RR) = 3.30; 95% CI = 1.46-7.44; p = 0.004]. Among mothers who developed premature rupture of membranes, those with a low CD4+ level had a similarly increased risk of vertical transmission (RR = 4.33; 95% CI = 1.78-10.5; p = 0.003). These relative risks were much higher than those for mothers who did not smoke after the first trimester (RR = 1.14; 95% CI = 0.48-2.70; p = 0.76) or have premature rupture of membranes (RR = 1.29; 95% CI = 0.61-2.74; p = 0.50), indicating that these factors modified the effect of CD4+ level on transmission. Among all mothers without regard to CD4+ level, those who experienced preterm premature rupture of membranes were also at greater risk of transmission (RR = 2.24; 95% CI = 1.07-4.69; p = 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)