The relations among race, family income, and low birth weight were examined using information obtained from the National Longitudinal Survey of Youth, which conducted yearly interviews with a nationally representative sample of young women identified in the late 1970s. Data were available for these women and their offspring from 1979 through 1988. Maternal education, maternal age, age/parity risk, marital status, and smoking during pregnancy served as covariates in cross-sectional and longitudinal analyses. The risk of low birth weight among births to black women and white women who were poor was at similarly high levels regardless of whether poverty was determined prior to study entrance or during the study period. Longitudinal analyses showed an exceptionally large increase in risk of low birth weight among children born to women whose prior pregnancy ended in a low-birth-weight infant. These two findings emphasize the importance of factors antecedent to the pregnancy in the genesis of low birth weight.