There generally are no obvious signs of Klinefelter syndrome until puberty. At this time, boys with Klinefelter syndrome often do not have the increase in testosterone levels that normally occurs.

Because of low testosterone levels, boys with Klinefelter syndrome may:

• Have sparse pubic, facial, and body hair.
• Have underdeveloped muscles.
• Have enlarged breasts (gynecomastia).
• Be taller than other males in their family and have long legs, narrow shoulders, and wide hips.

Men with Klinefelter syndrome have smaller-than-expected testicles, are generally infertile, and cannot father children without using special fertility techniques.

See an illustration of a male with Klinefelter syndrome.

Mental, emotional, and behavioral concerns

Males with Klinefelter syndrome appear to have reduced abilities in specific areas, including:¹

• Language development. Boys with Klinefelter syndrome usually have delayed or slowly developing speech skills and poor verbal skills.
• Critical thinking skills, problem solving, and ability to plan.
• Multi-tasking.
• Impulse control.
• Response time.

Some research shows that these problems with learning and behavior may be caused by the way the brain grows in males with Klinefelter syndrome.²

Boys may have emotional problems that range from being shy and immature to being overly anxious or aggressive. They may also have poor social skills, which may cause problems for them in school and in other social situations. They are at risk for developing psychiatric disorders, such as anxiety, depression, and drug and alcohol abuse.