Chromosomal abnormalities are an important cause of mental retardation. We studied the frequency of karyotype abnormalities in 74 mentally retarded patients selected from 306 patients referred to our clinic. Giemsa-banding was done on all cases. Additional studies in abnormal cases included autoradiography and X and Y chromatin. Karyotype analyses and blood group (Xg and Duffy) studies were carried out in family members in some cases. Fourteen of these children had chromosomal abnormalities, seven sex chromosomal, and seven had autosomal abnormalities. Three patients had 45,X and one had a 45,X/46,Xr(X) karyotype. Other sex chromosomal abnormalities were 46,XX/48,XXXX; 48,XXXY/49,XXXXY; and 48,XXYY. Autosomal abnormalities were 46,XX,1q-; 46,XY,2q-; 46,XY,5p-; 46,XY,dup(5p); 45,XX,t(13,14); and 46,XY,17p-. This is the first report from India of cytogenetic abnormalities in idiopathic mental retardation. The chromosomal studies in these patients help not only in accurate diagnosis, proper prognosis, and genetic counseling but also in gene localization and in the study of the origin of X-chromosome abnormalities.