What is XXY syndrome (Klinefelter syndrome)?

Klinefelter syndrome is caused by the presence of an extra 'X' chromosome, one of the sex chromosomes, in each of a male’s cells. Because individuals with an XXY chromosome arrangement have a Y chromosome, they are considered males, often not knowing they have an extra chromosome. Physical characteristics may appear around the time of puberty. Children and adults may be taller than average, and may have minor to moderate learning disabilities. Individuals with XXY syndrome are also more likely to develop certain medical conditions, including osteoporosis (weak bones), varicose veins, type 2 diabetes, and heart valve defects. As adults, they are just as likely as females to develop breast cancer and certain autoimmune disorders.

Men typically have low levels of the hormone testosterone, leading to low growth of facial and body hair, and underdevelopment of the testes. As adults, nearly all males with XXY are infertile.

How common is Klinefelter syndrome?

The condition occurs in 1 in 500 to 1 in 1000 male births. The condition is not related to race, nationality, religion or socioeconomic status. There is usually no family history of the condition.

How is Klinefelter syndrome treated?

Teens and adults can be treated with hormones. Many individuals with Klinefelter syndrome are typical males who receive no treatment. Those who wish to have children can often be helped with reproductive technologies.

Other names for

  • Klinefelter syndrome


  • Clin Pediatr. 2001 Dec;40(12):639-51.
  • Gardner, R. J. M., & Sutherland, G. R. (2011). Chromosome abnormalities and genetic counseling (4th ed.). New York: Oxford University Press, Inc.
  • Jones, K. L. (Ed.). (2013). Smith’s recognizable patterns of human malformation (7th ed.). Philadelphia: Elsevier Inc.
  • Orphanet J Rare Dis. 2006 Oct;24(1):42.