Not all boys with Klinefelter syndrome will have noticeable symptoms. Other boys can have symptoms that are physically apparent or ones that interfere with their speech, learning, and development. Babies with Klinefelter syndrome typically have weak muscles, reduced strength, and quiet personalities. They also can take longer to do things like sit up, crawl, walk, and speak. Compared with other kids their age, boys with Klinefelter syndrome might have some or all of the following symptoms:

-- weaker bones
-- a lower energy level
-- smaller penis and testicles
-- delayed or incomplete puberty 
-- less facial and body hair following puberty
-- a taller, less muscular body
-- broader hips and longer legs
-- larger breasts 

In addition to physical symptoms, many boys with Klinefelter syndrome will also show symptoms related to their development of social and language skills. They may have difficulty paying attention. A lot of boys learn to talk late or have trouble using words to express their emotions. They also can have trouble with things like learning to spell, read, and write.

Socially, boys with Klinefelter syndrome tend to have quiet, docile personalities. They rarely cause trouble and are often more helpful and thoughtful than other boys. They are often shy and sensitive, and many are less self-confident and less active than other boys their age. Since boys with Klinefelter syndrome can have problems with schoolwork and sports, they may feel like they don't fit in with other kids their age. But by the time they are men, most will have normal social relationships with friends, family members, and others.