We evaluated the effect of maternal magnesium sulfate treatment on selected neonatal outcomes in ≤1000-g infants. In a 1-year (1992-1993) observational study, the National Institute of Child Health and Human Development Network of Maternal-Fetal Medicine Units collected outcome data for 799 infants whose birth weights were ≤1000 g. Only singleton infants, with a gestational age >20 weeks who were not the product of an induced abortion were included. Our analysis was further limited to those infants without major congenital anomalies, who were deemed potentially viable by the obstetrician, whose mother would have undergone a cesarean delivery for fetal indications, and who survived greater than 2 days. Outcomes were compared in infants whose mothers did and did not receive magnesium sulfate for labor tocolysis. Among the 124 women who did and the 184 who did not receive magnesium sulfate tocolytic therapy, the frequencies of grade III or IV intraventricular hemorrhage (16 vs. 20%, p=0.34), seizure activity (7 vs. 10%, p=0.35), grade III or IV retinopathy of prematurity (21 vs. 18% p = 0.59), abnormal neurological exam (28 vs. 28%, p = 0.91), intact survival to 120 days or to discharge (48 vs. 44%, p=0.54), and infant mortality (23 vs. 31%, p = 0.10) were similar. Multiple logistic regression analysis was used to control for the effect of potential confounders (specifically, gestational age) and confirmed the lack of a significant association between maternal magnesium sulfate treatment for tocolysis and selected neonatal outcomes in this population of ≤1000-gram infants.