Mathematical analysis of prenatal and perinatal risk factors was performed on the first 681 published cases of idiopathic congenital cerebral palsy (born 1959-1970) in the west Swedish population-based cerebral palsy (CP) study. Analysis indicates that an estimated 40% of etiologically undiagnosed cases of CP in the community (48% of those born at term and 24% of those born prematurely) are genetically caused. These proportions of genetic causation are no less in CP than in idiopathic mental retardation. Genetic causes account for 60% of maturely born hemiplegics, 45% of maturely born spastic diplegics, 32% of premature spastic diplegics and virtually all cases of pure ataxia. About 23% of CP cases in the community have suffered nongenetic brain damage in accordance with the two-stage model. The residue of 37% is characterized by a single risk factor, usually perinatal.