In a study of the urban-rural distribution of 1014 low-grade mental defectives, institutionalized from 1940-1960 at the Vipeholm Hospital, Sweden, by place of residence, significantly more of patients come from rural areas than from urban areas. A similar significant urban-rural distribution difference was found in a control material comprising all low-grade mental defectives in Sweden, born in 1940[long dash]1944 and under institutional care during the years 1955[long dash]1959. Distribution of the severe defectives by place of residence was correlated to the relative size of the population living in population clusters. Thus, mental defectives significantly more often come from sparsely settled areas than from densely settled areas. Possibly this observed variation is an effect of selective migration. A study of distribution of patients by place of residence in parishes with different percentages of the economically active population engaged in either agriculture, forestry and fishing, or in mining, manufacturing, construction, etc., displayed no significant deviations when correction for population density was made.