Low birth weight, the primary predictor of infant mortality and morbidity, can be a result of shortened gestation (preterm delivery) or fetal growth retardation (small for gestational age). We examined the relationship between maternal characteristics and the risk of delivering a small-for-gestational age infant in 2228 women who participated in the University of California, San Diego Prenatal Nutrition Project between 1978 and 1988. A multivariate analysis indicated that significant risk factors for small for gestational age were cigarette smoking (odds ratio, 3.18), a low rate of maternal weight gain (odds ratio, 2.96), black ethnicity (odds ratio, 2.60), pregravid underweight (odds ratio, 2.36), Asian ethnicity (odds ratio, 1.88), primiparity (odds ratio, 1.85), and low maternal height (odds ratio, 1.63). These findings are contrasted with those previously published on preterm deliveries in the same cohort. We conclude that with the exception of black ethnicity and low maternal weight gain, different maternal characteristics were significantly associated with small-for-gestational-age and preterm birth in this population.