Black infants are born with low birth weights (less than 2500 g) and very low birth weights (less than 1500 g) at twice the rate of white infants. We investigated the effect of prenatal care delivered in a health maintenance organization on the birth weights of black and white infants at normal risk for low birth weight. Using birth certificates for all children born in 1978 in the California Kaiser-Permanente hospitals, we studied data on more than 31,000 black and white newborns whose mothers' ages and levels of education were comparable. The data show that black mothers used prenatal care less extensively and had a higher incidence of infants with low birth weights (8.4 vs. 3.6 percent) and very low birth weights (2.0 vs. 0.7 percent) than white mothers. The difference in the use of prenatal care, however, accounted for less than 15 percent of the difference in the incidence of low birth weight. The rates of low birth weight, very low birth weight, and preterm birth (less than 260 days' gestation) decreased with increasing levels of prenatal care for both blacks and whites. However, increasing levels of care were associated with a greater reduction among black infants than among white infants in low birth weight, very low birth weight, and low birth weight at term (greater than or equal to 260 days' gestation). When we compared mothers who received adequate care with those who received inadequate care, the relative risk of giving birth to a very-low-birth-weight infant was reduced 3.6-fold (95 percent confidence interval, 2.0 to 6.6) for black mothers and 2.1-fold (confidence interval, 1.3 to 3.4) for white mothers; the relative risk of giving birth to a low-birth-weight infant at term was reduced 3.4-fold (95 percent confidence interval, 2.2 to 5.4) for black mothers and 1.6-fold (confidence interval, 1.1 to 2.3) for white mothers. We conclude that even in a population of women at low risk for giving birth to low-birth-weight infants, prenatal care is more beneficial for blacks than for whites.