A large prospective study investigated prenatal and perinatal antecedents of chronic motor dysfunction (cerebral palsy [CP]), evaluating approximately 400 characteristics of the mothers, pregnancies, or deliveries. In addition to confirming some, but not all, of the classic risk factors for CP, this study observed relatively large increases in the CP rate in association with maternal mental retardation, seizure disorders, hyperthyroidism, or with the administration of thyroid hormone and estrogen in pregnancy. Some risk factors were predictive of CP only insofar as they were associated with low birth weight or low Apgar scores. Among factors not significantly related to CP rate were maternal age, parity, socioeconomic status, smoking history, maternal diabetes, first trimester vaginal bleeding, kidney or bladder infection, moderate hypertension, long cord, use of anesthetic agents, or use of oxytoxics for initiation or augmentation of labor. Duration of labor, whether precipitate or prolonged, was not a risk factor for CP.