The prevalence of severe mental retardation, derived from military records, is analyzed in terms of time trend, region, religious affiliation and urbanization of birth place. The data are singular in that they are national and virtually complete for a total population of young men. 19-year-old survivors of male births in the Netherlands over the years 1944 through 1947. and they include variables not previously studied. Previous local studies are shown to agree well with the national prevalence rate of 3.7 per 1,000. Prevalence rose through the four birth years for all forms of severe mental retardation, but most markedly for Down's syndrome. In this condition agency surveys usually yield equivalent rates to population surveys, and are an economical means of monitoring prevalence. Differences in rates are quite likely to indicate substantive differences. The remarkable similarities in rates from past surveys may reflect a high proportion of chromosomal and genetic abnormalities. The divergence between urban and rural rates over time demonstrated in this paper for four annual cohorts is attributed by inference to disparate increases in survival in different ecologic settings.