From a population-based series of 773 patients with cerebral palsy (CP) born in 1959-78, an analysis was made of the epidemiological trends over this period of 20 years, divided into five 4-year periods with emphasis on the last two. After a significantly decreasing incidence of CP in the first three periods (1959-70) from 1.9 to 1.4 per thousand, there was a significant increase in the last two periods, reaching 2.0 per thousand in the period 1975-78. Both the decreasing trend in the earlier periods and the increasing trend in the more recent ones were mainly referrable to spastic/ataxic diplegia in preterm CP, and to dyskinetic syndromes in CP infants born at term. With respect to pathogenesis, the corresponding changes in CP incidence were mainly accounted for by the group with potential perinatal risk factors. When analysed on the basis of surviving babies in birth-weight-specific groups, the incidence of CP in 1971-78 was found to have increased in all groups, but this was only statistically significant in the low birth weight group of 2 000-2 500 g. Changing trends in incidences ran parallel with a steadily progressive decline in perinatal mortality through all five periods. A considerable and cumulative net gain of surviving non-CP children was continuously achieved; this was also true for 1970-78, in spite of an increasing CP morbidity during these last two 4-year periods.