To examine the impact of demographic shifts and changes in perinatal medicine on the distribution of cerebral palsy (CP), we investigated characteristics of affected children in a large, recent population-based American cohort study. Children with moderate or severe congenital CP born in four northern California counties in 1983 through 1985 and surviving to age 3 years were identified through records of state service agencies and clinical examination or record review by a single physician. We compared information from birth certificates for 192 children with CP and 155,636 survivors without CP born in those counties in the same period. Children with birth weights < 2500 gm contributed 47.4% of the CP in this population; those < 1000 gm, who were 0.20% of survivors, contributed 7.8%. Children with birth weights of 4000 to 4500 gm were at lowest risk. Among singletons, prevalence of CP was lowest (0.92/1000) in infants born to women aged 25 to 34 years, and was significantly higher in children whose mothers were 40 years or older (3.3/1000), especially if they were high in parity (6.9/1000). Children of teenaged mothers or fathers were at somewhat increased risk of CP. Early gestational age at birth was also an important independent risk factor. Prevalence of CP was slightly higher in black children, apparently related to a greater tendency to be low in birth weight. The time during pregnancy when prenatal care began was similar for children with CP and for the general population. For the 95% of children born weighing > or = 2500 gm, birth in a hospital lacking a special care nursery was not associated with increased risk of CP. Almost 8% of CP occurred in children born weighing < 1000 gm, a group that produced few survivors in the past; 28.1% occurred in children born weighing < 1500 gm. Neither early initiation of prenatal care nor, for that large majority of neonates weighing > 1500 gm, delivery at a hospital with specialized facilities was associated with a lower risk of CP.