Nonspecific mental retardation (i.e., mental retardation with no known etiology) at all levels of functioning was investigated in the population of British Columbia, with ascertainment through the British Columbia Health Surveillance Registry. The age-specific prevalence of nonspecific retardation for the 15- to 29-year age group, where ascertainment was best, was 4.4 per 1,000 population at year-end 1978. A minimum incidence of 5.2 per 1,000 male live births and 4.0 per 1,000 female live births was calculated for the 1952-1965 birth cohort. Data on the number of cases associated with either microcephalus, hydrocephalus, epilepsy, or cerebral palsy were presented. Seventy-three percent of the cases with mild mental retardation had no associated disability, whereas only 26.9 percent of profoundly retarded cases were without any of these conditions. Survival rates varied with respect to the level of mental retardation.