Discovery of 49 XXXXY syndrome in a six years young boy allows description of the main clinical characteristics of this disease: hypotrophy, facial anomalies, hypogenitalism, delayed speech development and oligophrenia. Radio-cubital synostosis is quite specific in this syndrome. The hypothesis of a correlation between clinical anomalies and excess of genes induced by polysomia has been suggested. We give results of five X-linked enzymatic activities: steroid sulfatase (STS) (located on the probably noninactivated segment), Hypoxanthine Guanine Phosphoribosyl Transferase (HGPRT), Glucose 6 Phosphate Déshydrogenase (G6PD), Phospho Glycerate Kinase (PGK), Alpha Galactosidase A (Alpha GAL A). Only STS activity seems to be significatively increased.