Background: Based on standard social benefit registers, the prevalence of intellectual disability (ID) in Finland is estimated to be 0.6%, while epidemiological surveys yield 1.1%. Combining several registers, our aim was to find a more reliable estimate of the prevalence of ID, especially among children and adolescents. This is important when special or inclusive general services are planned to meet the various needs of people with ID. Method: A survey based on eight national health and social benefit registers. Results: Combining different registers yielded a mean ID prevalence of 0.70% (95% CI 0.69-0.70%), with marked differences according to sex and age group (range 0.38-0.96%). Capture-recapture analysis gave higher prevalence estimates (range 0.57-1.08%). Conclusions: When several health and social benefit registers are surveyed, the estimated prevalence of ID increases, approaching that obtained in epidemiological surveys.