Sex chromosome abnormalities occur in at least 1 in 400 births and include the well-described 47,XXX, 47,XXY, 47,XYY, and 45,X karyotypes. The addition of more than one extra X or Y chromosome occurs rarely, and little information is available in the medical literature. Individual case reports make up most of this body of knowledge, and all are based on subjects who identified themselves postnatally. Many were ascertained through screenings of institutions and hospitals; thus, there is no unbiased information on the natural history of poly X and Y karyotypes. A direct relationship between the number of additional sex chromosomes and the severity of the phenotype is generally assumed. The purpose of this article is to summarize what is known about these conditions and to present 10 additional cases. The karyotypes include, 48,XXXX, 49,XXXXX, 48,XXYY, 48,XXXY, 49,XXXXY, 49,XXXYY, 48,XYYY, 49,XYYYY, and 49,XXYYY.