Prevalence studies of mental subnormality among regional populations are few. Detailed epidemiological enquiries have been carried out in the regions of Northern Ireland (Scally and MacKay, 1964), Wessex (Kushlick, 1965) and, for severe mental subnormality in children, in Middlesex and London (Goodman and Tizard, 1962). The North-East Region of Scotland is especially suitable for epidemiological studies in psychiatric morbidity by reason of its geography, demography and centralization of psychiatric services (Innes and Sharp, 1962; Baldwin and Millar, 1964). A Regional Psychiatric Case Register giving rapid access to social and clinical information on all patients is fully operational (Baldwin et al., 1965). Using this facility as a primary source of data, a prevalence study of mental subnormality among the half million population of the North-East of Scotland was carried out. The objects of the study were three-fold: first and primarily, to establish case rates for known subnormality, examine patterns of morbidity and the factors that influence this and to compare these with the findings of similar epidemiological studies; second, to gain operational data relevant to existing practice, needs and planning of services; third, to provide a data bank within the Regional Psychiatric Case Register for mounting detailed clinical, genetical, social or biochemical studies on an area population of subnormal patients.