Klinefelter syndrome (KS), also known as 47, XXY is the set of symptoms that result from two or more X chromosomes in males. The primary features are infertility and small poorly functioning testicles. Often, symptoms are subtle and subjects do not realize they are affected., symptoms are more evident and may include weaker muscles, greater height, poor coordination, less body hair, breast growth, and less interest in sex. Often it is only at puberty that these symptoms are noticed.  Intelligence is usually normal; however, reading difficulties and problems with speech are more common. Symptoms are typically more severe if three or more X chromosomes are present (48,XXXY syndrome or 49,XXXXY syndrome).

Klinefelter syndrome occurs randomly. The extra X chromosome comes from the father and mother nearly equally. An older mother may have a slightly increased risk of a child with KS. The condition is not typically inherited from a father with the syndrome. The syndrome is defined by the presence of at least one extra X chromosome in addition to a Y chromosome yielding a total of 47 or more chromosomes rather than the usual 46. KS is diagnosed by the genetic test known as a karyotype.

While no cure is known, a number of treatments may help. Physical therapy, speech and language therapy, counselling, and adjustments of teaching methods may be useful. Testosterone replacement may be used in those who have significantly lower levels. Enlarged breasts may be removed by surgery. About half of affected males have a chance of fathering children with the help of assisted reproductive technology, but this is expensive and not risk free. XXY males appear to have a higher risk of breast cancer than typical, but still lower than that of females. People with the condition have a nearly normal life expectancy.

Klinefelter syndrome is one of the most common chromosomal disorders, occurring in one to two per 1,000 live male births. It is named after American endocrinologist Harry Klinefelter, who identified the condition in the 1940s. In 1956, the extra X chromosome was identified as the cause. Mice can also have the XXY syndrome, making them a useful research model.

Signs and symptoms include:

longer than average to sit up, crawl and walk

Delay in speaking

Problems at birth, such as testicles that haven't descended into the scrotum

Boys and teenagers

Taller than average stature

Longer legs, shorter torso and broader hips compared with other boys

Absent, delayed or incomplete puberty

After puberty, less muscle and less facial and body hair compared with other teens

Small, firm testicles

Small penis

Enlarged breast tissue (gynecomastia)

Weak bones

Low energy levels

Tendency to be shy and sensitive

Difficulty expressing thoughts and feelings or socializing

Problems with reading, writing, spelling or math