How many people are affected by or at risk for Klinefelter syndrome (KS)?

Researchers estimate that 1 male in about 500 newborn males has an extra X chromosome, making KS among the most common chromosomal disorders seen in all newborns.1 The likelihood of a third or fourth X is much rarer: 2

Prevalence of Klinefelter syndrome variants

Number of extra X chromosomes One (XXY) Two (XXXY) Three (XXXXY)
Number of newborn males with the condition 1 in 500 1 in 50,000 1 in 85,000 to 100,000

Scientists are not sure what factors increase the risk of KS. The error that produces the extra chromosome occurs at random, meaning the error is not hereditary (pronounced huh-RED-i-ter-ee) or passed down from parent to child. Research suggests that older mothers might be slightly more likely to have a son with KS. However, the extra X chromosome in KS comes from the father about one-half of the time.3


  1. Nielsen, J., & Wohlert, M. (1991). Chromosome abnormalities found among 34,910 newborn children: Results from a 13-year incidence study in Arhus, Denmark. Human Genetics, 87(1), 81–83.
  2. Klinefelter, H.F., Reifenstein, E.C., & Albright, F. (1942). Syndrome characterized by gynecomastia aspermatogenesis without A-Leydigism and increased excretion of follicle stimulating hormone. Journal of Clinical Endocrinology & Metabolism, 2, 615–627.
  3. National Human Genome Research Institute. Learning about Klinefelter Syndrome. Retrieved on June 5, 2012 from

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