A cytogenetic survey was carried out on 449 patients (261 males and 188 females) in an institution for the mentally retarded in Japan. A total of 37 patients (8.1%) were shown to have chromosome abnormalities. There were 33 individuals (7.3%) with 21 trisomy. In addition, we found one patient with 46,XY/47,XY,+12p, one with 46,XY,r(22), and one with 45,XY,-13,-14,+t(13q14q). Only one female was found to have an abnormal sex chromosome constitution, 47,XXX. The significant contribution of chromosome abnormalities in the etiology of mental retardation is also shown in the present survey. The most common chromosome abnormality was 21 trisomy, as seen in other similar surveys.