The authors did chromosomal studies on four hundred and forty patients in an institution for the mentally retarded. Among the eighty-eight patients who had Down's syndrome, five had a reciprocal translocation, for a frequency of 5.6 per cent. The parents of four of these children had a normal chromosomal pattern. Among the remaining three hundred and fifty-two patients, two had a gonosomal anomaly and three had an autosomal anomaly, for a frequency of 1.4 per cent. These data are compatible with the nature of the psychomotor defects observed and the mean age of the patients who constituted a heterogeneous group of mentally retarded individuals.
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