Patterns in the incidence of cerebral palsy are described over a 20-year period in Western Australia. The incidence rose to a peak between 1966 and 1970, then fell again. This pattern was particularly marked in the spastic syndromes and was seen in both sexes, in each maternal age and parity group, in each IQ group, in both multiple and single births, and in infants born in metropolitan (but not rural) areas. Improvement was more marked in heavier than in lighter infants. Since 1968 the male rate has fallen more quickly than that for females. The risk of cerebral palsy with high maternal age declined, but it remained high in relation to high parities. There were marked reductions in the proportions of older and higher-parity mothers in Western Australia over the study period, and in the proportion of multiple births. There was also a shift toward heavier babies from 1968 to 1975. Social-class information was not available. The data indicate that factors in addition to changes in perinatal care were operating to improve neonatal outcome in terms of long-term handicap in Western Australia.