One of the common clinical interests shared by physicians and psychiatrists is the genetically determined syndrome to which Klinefelter has given his name. Over the past few years positive identification has become possible through advances in cytogenetics, and more recently evidence has been accumulating which seems to indicate that a significant correlation may exist between Klinefelter's Syndrome and the development of other diseases, malignant disease in particular. Mental retardation is one of the most common symptoms found in the syndrome, so the psychiatrist is often in the best position to detect it. The case to be described here as well as having a few interesting facets of psychopathology may also provide more evidence of the pathological loading which the basic genetic abnormality seems to carry.