To identify the risk factors responsible for differences in birth weight between blacks and whites, we investigated the effects of four maternal characteristics (age, parity, marital status, and education) on rates of very low birth weight (less than 1500 g) and moderately low birth weight (between 1500 and 2500 g). Using 1983 national data, we found that the black:white rate ratio was 3.0 for very low birth weight and 2.3 for moderately low birth weight. The four maternal factors had directionally similar but quantitatively different effects on very low and moderately low birth weight among blacks and whites. Furthermore, the racial differences in infants' birth weights were greater among low-risk than among high-risk mothers, especially for very low birth weight (black:white ratios of 3.4 and 1.7, respectively). We also examined secular trends in the rates of low birth weight among blacks and whites. Between 1973 and 1983, births of infants with moderately low birth weights decreased more among whites than among blacks, whereas births of infants with very low birth weights increased among blacks and decreased among whites. Fifteen percent of the decline in the rate of moderately low birth weight among whites could be attributed to favorable changes in maternal characteristics (primarily an increase in educational level). Among blacks, adverse changes in maternal characteristics (primarily an increase in births to unmarried women) accounted for 35 percent of the increase in the rate of very low birth weight. The adverse effects of childbearing by teenagers on the outcome of pregnancy among blacks appear to have been overemphasized. The persistence of large racial differences in birth weight, even among mothers at low risk, emphasizes the need for specific targeted interventions to achieve further improvements in infant health.