A study of cerebral palsy in multiple births was undertaken to test genetic involvement and assess the impact of the special conditions of pregnancy and parturition in these cases. Complete ascertainment of cerebral palsy in multiple gestations that occurred in Western Australia between 1956 and 1985 was obtained from the Western Australian Cerebral Palsy Register. There were 74 twins and 5 triplets. Data on sex, birth order, motor handicap, outcome in co-twins and triplets, zygosity, and pedigree information was obtained from the Register, hospital records, and, where possible, by interview of the parent(s) of the propositi. There was a significantly higher (P = 0.0026) concordance rate in MZ than in DZ twin pairs. However, pedigree studies showed no other relatives with a motor handicap similar to that of the propositi. This is consistent with a multifactorial cause in at least some of the cases. The sex ratio of affected twins was found to be 2.1 compared to 1.3 for singletons and all 5 affected triplets were boys. The trend of increasing sex ratio with increasing plurality was significant at the 1% level.