The object of this case-control study was to investigate and analyse perinatal risk indicators of mental retardation. Data concerning 33 perinatal factors possibly related to mental retardation were collected retrospectively for the population of an area in Finland. The subjects were 339 cases born between 1967 and 1981 with mental retardation of suspected perinatal or unknown aetiology and 364 controls born in the same period. Independent risk indicators were low maternal social class, multiparity, multiple pregnancy, male sex, being born small for gestational age, asphyxia, hypoglycaemia and hyperbilirubinaemia in the newborn infant. Perinatal problems increased the risk for multiple disabilities. Preterm birth under 33 gestational weeks and very low birthweight (< 1500 g) were associated with considerable risk, but neither was an independent risk indicator. The introduction of neonatal intensive care seems to have altered the risk of death to the risk of surviving with disability.