The incidence of minor malformations in a group of 137 adults with cerebral palsy, living in a New York State residential institution, is reported from the Division of Developmental Pediatrics, Robert Warner Rehabilitation Center, State University of New York, Buffalo, NY. Subjects were classified in three etiological groups: (1) prenatal onset, (2) postnatal onset, and (3) perinatal onset. Criteria for enrollment in group 1 included a known prenatal cause (i.e., CNS malformation, chromosomal aberration or congenital infection) or fullterm birth with unremarkable delivery and neonatal course. Of 109 patients with CP of prenatal origin, 82% had unknown causes, 10% CNS malformation, 5% chromosomal aberration and 3% congenital infection. Of 28 patients with postnatal causes, 39% had bacterial meningitis, 32% encephalitis, 11% metabolic disease, 7% cardiopulmonary arrest and 7% trauma. Spastic quadriplegia, profound mental retardation, and seizures occurred in 80-90% of the total sample, and the neurological findings were similar in the two groups. The prenatal group with known etiology and a subgroup with unknown etiology had significantly more minor malformations than the postnatal group. Individuals with CP and multiple minor malformations, in the absence of definitive postnatal or perinatal factors, have CNS developmental dysfunction caused most likely by prenatal influences.