Birth weight, gestational age at delivery, and other factors were collected for 171 white children with spastic cerebral palsy. Their birth weights were compared with the birth weight distribution expected for a population of the same race, gestation, sex, maternal height, and parity, born in the same geographic area, and during the same time period. Birth weights of children with spastic cerebral palsy tended to be significantly lower than the median birth weight of their comparison population. Analysis stratified by gestation at delivery suggested that if the reduced birth weight were causally associated with the spastic cerebral palsy, 22% of cases were attributable to being below the 10th percentile of the comparison population birth weight distribution. The risk of spastic cerebral palsy associated with poor intrauterine growth was dependent on gestation at delivery; poorly grown infants delivered between 34 and 37 weeks' gestation were at highest risk. Some probable pathways by which growth retardation could result in brain damage (intrapartum hypoxia, hypoglycemia, and hypothermia) were investigated. Only intrapartum hypoxia may have played a causal role but probably accounted for less than 2% of all cases. These data suggest that spastic cerebral palsy is associated with poor intrauterine growth in infants of more than 33 weeks' gestation, but no important causal mechanism has yet been identified.