In a retrospective review of cases, the causes of cerebral palsy in 142 children living in the northwestern United States and born after Jan 1, 1970, were prenatal in 50% of the cases, similar to findings recently reported from Sweden. The causes were perinatal in 33%, postnatal in 10%, and mixed in 7%. Epidemiologic studies from the 1950s report the origins of cerebral palsy to be mostly perinatal. The discrepancy is explained largely by differences in interpretation of medical findings prompted by recent advances in knowledge of fetal development and implications of obstetric and neonatal events. Cerebral palsy was considered medically preventable in 6% of full-term perinatal cases, possibly medically preventable in another 4%, and socially preventable in 4% of posttraumatic and 22% of preterm cases. Research in prenatal development, improvements in health education, and expansion of clinical genetic services are suggested as the most promising avenues toward prevention of cerebral palsy.