The purpose of this study is to identify obstetrical factors associated with adverse neurological outcome in < or =1000-g infants. In a 1-year (1992-1993) observational study, the NICHD MFMU Network collected obstetrical risk factors for 486 infants who weighed < or =1000 g at birth and who survived > 2 days. Infants' records were abstracted for seizures, intraventricular hemorrhage, and an abnormal neurological evaluation. Seventy-nine (16%) infants had a Grade III or IV intraventricular hemorrhage, 46 (9%) developed seizures and 57 (14%) had an abnormal neurological evaluation. Both lower birth weight and earlier gestational age correlated (P <0.01) with an increasing incidence of all three outcomes. Several other factors appeared to be associated with neurological morbidity, however, after controlling for potential confounders in the multivariate analyses, most of these factors were no longer significant. African-American race, odds ratio (OR) 0.6 (0.3-1.0), and severe preeclampsia, OR 0.2 (0.1-0.7), were protective against intraventricular hemorrhage. Maternal treatment with corticosteroids did not impact neurological outcome in this study population. We conclude that, in a population of < or =1000-g infants, lower birth weight and earlier gestational age were the only consistently significant predictors of all three adverse neurological outcomes.