Sex chromosome abnormalities, such as 47,XXX, 47,XXY, 47,XYY, and 45,X, are relatively common and occur in approximately 1 of 400 births. Sex chromosome tetrasomy and pentasomy are much rarer events. The somatic and developmental consequences of supernumerary sex chromosomes have not been studied adequately. This is especially true of individuals with only supernumerary Y chromosomes. Based on available case reports, the effects of extra Y chromosomes appear not as severe as those of supernumerary X chromosomes. Only two case reports of nonmosaic tetrasomy of the Y chromosome have been published. We evaluated a 30-year-old man with a 49,XYYYY karyotype and assessed his severe physical and mental handicaps (particularly the endocrine abnormalities) and attempted to clarify the effects of extra Y chromosomes on growth, development, and behavior.