The objective of this study was to determine whether a relationship exists between adequacy of prenatal care and preterm delivery, low-birth-weight, and small-for-gestational-age infants. Data from 1771 patients enrolled in an ongoing study of maternal growth in young gravidas were studied. The indices of both Kessner and colleagues (the Kessner index) and Kotelchuck (the Kotelchuck index) were used to determine adequacy of prenatal care. Patients receiving adequate care were compared with those receiving intermediate or inadequate care and the incidence of preterm birth, low-birth-weight, and small-for-gestational-age infants was determined in each group. The study controlled for potential confounding variables, that is, black ethnicity, maternal age, pregravid body mass index, parity, adequate prenatal weight gain, smoking, and previous delivery of low-birth-weight or preterm infant. The data were analyzed by logistic regression. When classified according to the Kessner index, 290 (16.4%) of the women received inadequate care, 961 (54.3%) received intermediate care, and 520 (29.4%) received adequate care. By the Kotelchuck index, 651 (36.8%) of the women received inadequate care, 201 (11.3%) received intermediate care, 604 (34.1%) received adequate care, and 315 (17.8%) received adequate-plus care. By use of the Kessner index and by estimating gestation from the last menstrual period, it was determined that women who received inadequate care had a 2.8 times greater risk (95% confidence interval [CI], 2.07-3.78), and by use of the Kotelchuck index, a 2.1 times greater risk (95% CI, 1.58-2.81) of having a preterm delivery. When the obstetric estimate of gestation was used, risk was 2.01 times greater (95% CI, 1.44-2.80) with the Kessner index and 1.4 times greater (95% CI, 1.0-1.94) with the Kotelchuck index. There was little effect of prenatal care on incidence of small-for-gestational-age infants. The results of this study confirmed that inadequate prenatal care is associated with an increased risk of preterm delivery.