In a population-based study cerebral palsy was diagnosed in 110 cases (2.4 per 1,000) among live born children with birth weight > or = 500 g (N = 45,976) during the 20-year period 1970-89 (cerebral palsy cases with a postneonatal etiology excluded). The incidence of cerebral palsy showed a linear declining trend from 2.8 per 1,000 in the first five-year cohort born 1970-74 to 2.0 per 1,000 in children born 1985-89 (p = 0.17). 15.9% of the decline in incidence of cerebral palsy from the first to the second ten-year cohort could be explained by a decreasing rate of low birth weight (500-2,499 g) in the population, from 4.2% 1970-79 to 3.8% 1980-89 (p