The California Cerebral Palsy Project (CACP) is a population-based study of 192 children with moderate or severe congenital cerebral palsy who were born between 1983 and 1985 in four San Francisco Bay area counties and who were alive and residing in California at age 3 years. Initial ascertainment of cases was based on records of two agencies known to enrol virtually all CACP-eligible children. Final case status was established by standardised clinical examination in 67% of cases and extensive record review in 33%. The 192 cases gave a prevalence at age 3 of 1.23/1000 survivors. Twins were 10% of the cases with a prevalence of 6.7/1000. Overall, 53% of the cases had birthweight greater than or equal to 2500 g and 28% had birthweight less than 1500 g. There was no association between birthweight and severity of functional impairment and no consistent association between birthweight and the presence of associated disabilities. The CACP prevalence is lower than that reported in other studies and is believed to be due to the more stringent case inclusion criteria employed for this research data base.