A survey of the sex-chromatin pattern in 1,252 males with mental deficiency revealed 10 apparent males with positive sex-chromatin patterns and with IQs ranging from 22 to 66. Nine of these are postpubertal and have findings characteristic of Klinefelter's syndrome. One patient is prepubertal and sexually immature. One of the patients has a few features suggesting mongolism. One patient is a twin; the other twin died at birth. He has an older halfbrother who has mongolism. Another patient has twin siblings, one of whom has mongolism. Recent reports of similar surveys confirm that about 1% of mentally deficient males have Klinefelter's syndrome. However, our data indicate the incidence is lower than that among the severely retarded and rises two- or threefold above that among the mildly retarded. The best control data indicate the incidence in the general population is about 0.2% or less. On the basis of these data, it appears that Klinefelter's syndrome with positive sex-chromatin pattern is likely to be associated with some degree of mental deficiency. The relation of the brain disorder to the anomalous sex-chromatin pattern is unknown.