In a retrospective study of 18,631 deliveries among women of low income, we examined the association of racial disparities in mean birth weight with population differences in maternal demographic characteristics and antepartum-intrapartum medical complications. The study population consisted of inborn, nonreferred, singleton, low-income patients delivered on the nonprivate service after at least five prenatal care visits. Repeat cesarean sections were not included. The mean birth weight for black infants was 214 g less than that for white infants. Black and white mothers differed significantly in marital status, age, and years of education. Black and white mothers also differed significantly in the incidence of chronic hypertension, preeclampsia-eclampsia, anemia, amnionitis, fever on admission, and sexually transmitted diseases. In this population, controlling for maternal demographic characteristics and medical complications of pregnancy produced a predicted mean birth weight for black infants that was 100 g less than that for white infants (53% of the observed racial difference in mean birth weight).