A cytogenetic survey of an unselected group of mentally retarded persons in a geographically limited area was made in order to establish the frequency of chromosome abnormalities within this group. In the area there were 2,157 patients, both institutionalized and living at home. A chromosome analysis was performed in 1905 cases; 359 (18.8%) of these had a chromosome anomaly: 281 (14.7%) Down's syndrome, 45 (2.4%) autosomal anomaly other than Down's, and 33 (1.8%) sex chromosome anomaly. The frequency of chromosome aberrations in relation to the whole group was higher compared with similar surveys, which leads to the conclusion that the total number of mentally retarded persons living within a limited area is determined by many factors. This means that a comparison between frequencies of abnormal chromosomes in an unselected group of mentally retarded persons is only relevant when related to the whole population living in the investigation area.