The health disadvantages of infants with teenage mothers are well documented. Because poor and minority women are disproportionately represented among teen mothers, differences in infant health by maternal age may reflect family background (pre-childbearing) characteristics rather than the effects of maternal age. To control for differences in family background, the authors compared birth outcomes and maternal behaviors that could affect fetal or infant health among sisters in the US National Longitudinal Survey of Youth (1979-1988). They compared sisters who had first births at different ages in order to study the relation between maternal age and low birth weight, prenatal care, smoking and alcohol use during pregnancy, breast feeding, and well-child visits. The authors found evidence that maternal family background accounts for many of the health-related disadvantages of the firstborn infants of teenage mothers. The findings suggest that disadvantaged black primiparous women in their twenties may be an important and possibly underemphasized target population for interventions designed to reduce excess black low birth weight and infant mortality rates.