OBJECTIVE: To examine the relationship between prenatal magnesium sulfate exposure and the risk for cerebral palsy (CP) or mental retardation (MR) among very low-birth-weight (VLBW; <1500 g) children. Secondarily, to investigate the effect of prenatal magnesium sulfate exposure on VLBW infant mortality. DESIGN: Cohort study with follow-up to 1 year of age; a subset followed up to 3 to 5 years. SETTING: Twenty-nine Georgia counties, including the 5-county Atlanta metropolitan area. PARTICIPANTS: All VLBW births (N=1097) occurring during 2 years (1986-1988); all metropolitan Atlanta VLBW neonates who survived infancy (N=519). MAIN OUTCOME MEASURES: Infant mortality as determined from vital statistics records. Development of CP or MR by 3 to 5 years of age among metropolitan Atlanta VLBW survivors as determined from the Metropolitan Atlanta Developmental Disabilities Surveillance Program. RESULTS: For the entire cohort, there was no association between prenatal magnesium sulfate exposure and infant mortality (adjusted rate ratio, 1.02; 95% confidence interval [CI], 0.83-1.25). Among Atlanta-born survivors, those exposed to magnesium sulfate had a lower prevalence of CP or MR than those not exposed (CP: magnesium sulfate, 0.9%, no magnesium sulfate, 7.7%, crude odds ratio [OR], 0.11, 95% CI, 0.02-0.81; MR: magnesium sulfate, 1.8%, no magnesium sulfate, 5.8%, crude OR, 0.30, 95% CI, 0.07-1.29). Multivariable adjustment had no appreciable effect on the ORs for CP or MR, but the CIs included 1.0. CONCLUSIONS: A reduced risk for CP, and possibly MR, among VLBW children is associated with prenatal magnesium sulfate exposure. The reduced risk for childhood CP or MR does not appear to be due to selective mortality of magnesium sulfate-exposed infants.