In a large prospective study, we investigated predictors of moderately low (1,501 to 2,500 g) and very low (less than 1,501 g) birth weight. Maternal age, height, and socioeconomic status, related to moderately low birth weight on univariate analysis, were not on multivariate analysis significant predictors for either low-birth-weight outcome. The leading predictors of very low birth weight were low weight (less than 2,000 g) of last live birth, fetal malformation, nonwhite race, early vaginal bleeding, cigarette smoking, and chorionitis. Some of these were also predictors of moderately low birth weight. About 20% of births under 1,501 g could be attributed to chorionitis, a characteristic of less than 3% of the births. Of the major predictors of low birth weight, only prolonged rupture of membranes, chorionitis, and congenital malformations contributed to the risk of cerebral palsy beyond their contribution to the risk of low birth weight.