It is done a cross-sectional study of the prevalence of mental retardation in the province of Jaén (Spain), based on the cumulated cases reported to the Institute of Social Services of the Social Security (INSERSO) until January 1, 1984. The main objective of this research is to know the prevalence of mental retardation in our province, and to analize its relationship with some sociodemographic variables. The prevalence obtained has been 4.09%, being a 59.27% of the prevalence due to endogenous/unexplained etiology. Mild subnormality constitutes a 8.4% of cases, being closely related with the size of the locality (municipio). Prevalence of mental subnormality showed a significative linear association with the size of the locality (r = -0.904, p less than 0.05), and with the population growth (r = -0.929, p less than 0.01). This relationship was not observed neither with the level of per capita income nor the altitude of the locality. A multiple regression analysis is made for every etiology of mental retardation and several social and geographic variables. We conclude that the figures for mental retardation in our province show a direct relationship with local development parameters.