From an unselected series of 560 cases of cerebral palsy in Sweden, born 1954-70, the changes in incidence through the four periods 1954-58, 1959-62, 1963-66 and 1967-70 were analyzed. The total incidence successively and significantly decreased from 2.2% in the first period to 1.3% in the last. This decrease was mainly related to (1) the syndromes of spastic and ataxic diplegia, (2) the low birth weight babies, and (3) the group with perinatal causes. The decrease was not related to any special intelligence quotient or geographical region.