Low birth weight is a major public health problem in the United States, contributing substantially both to infant mortality and to childhood handicap. The principal determinant of low birth weight in the United States is preterm delivery, a phenomenon of largely unknown etiology. Preterm delivery is more common in the United States than in many other industrialized nations, and is the factor most responsible for the relatively high infant mortality rate in the United States. Within the United States, Asian populations experience the lowest preterm delivery rates, while Hispanic and Native American populations experience slightly higher preterm delivery rates than the white population. African Americans, however, have much higher rates of preterm delivery than any of the other major ethnic groups. Poverty is strongly and consistently associated with low birth weight, but the precise social and environmental conditions that produce preterm delivery have not been elucidated. Although it is popular to link illicit drug use to low birth weight, a high low birth weight rate was characteristic of the United States for decades before the cocaine epidemic of the 1980s. Neither the low birth rate nor the preterm delivery rate has improved in the United States in the past quarter century. Most efforts to prevent prematurity or low birth weight, when carefully evaluated, have not proven effective. A major goal of biomedical research ought to be better understanding of the causes of this important public health problem.