In a unselected series of children born in 1959--70 with severe mental retardation (MR)---defined as IQ less than 50---in a Swedish county, the incidence, prevalence, gestational length, birth weight and associated CNS handicaps were analysed. The cumulative incidence at 1-16 years of age was calculated at 3.3% and the prevalence at 11-16 years at 2.8%. These figures were lower than in most other previous studies. In the great majority of cases the pathogenesis was of prenatal origin. The mean gestational lengths and birth weights were decreased compared with those of an average Swedish population. Severe MR affected large and small for date babies more often than could be expected. On the other hand, babies, with a low birth weight, appropriate for gestational age, and with an uncomplicated history, were found not to run a special risk of severe MR. Among the 122 children, 42% had one or more associated CNS handicaps---epilepsy (30%) and cerebral palsy (18%) being the most common.