The prevalence and origin of cerebral palsy in children born between 1983 and 1986 are reported. The crude live-birth prevalence was 2.49 per 1000; 1.56 for term births, 0.93 for preterm births. The increasing trend from 1970 persisted and was statistically significant for both groups. The most pronounced increase during 1983-1986 occurred in term cerebral palsy. The increasing use of neuroimaging techniques made the timing of damaging events more reliable. In term cerebral palsy, the origin was convincingly prenatal in 28% of cases and perinatal in 25%. The particular critical period for the brain damage underlying cerebral palsy was considered to be weeks 26-34 of gestation, i.e. when periventricular structures are extraordinarily vulnerable. In term infants, this occurs during late intrauterine life, while in the majority of preterms it occurs in early neonatal life. In total, two-thirds of cerebral palsy lesions might have been acquired during these decisive months of brain development.