The Klinefelter's Syndrome symptoms are determined by the extra X chromosome in the patient's kariotype (the patient's kariotype will be 47 XXY, not 46 XY, like in normal men) . In many men this disease is asymptomatic but it represents one of the most common genetic causes of male sterility.

The newborns and babies with Klinefelter's syndrome have mild hypotonia, reduced muscular strength and may have a slightly slower motor development. These symptoms may go unnoticed, and Klinefelter's syndrome is suspected only if the child has other disorders such as cryptorchidism (the testis's failure to descend into the scrotum), scrotal or inguinal hernias or hypospadias (uretral birth defect) occur. During childhood, the children with this disorder tend to be taller than the children of their age, thin or with a tendency towards obesity, they may have less muscular coordination than the other children, have difficulties in learning how to write and read and their speech skills develop slower.

The symptoms of Klinefelter's syndrome become more marked after the onset of puberty. Because of the reduced amount of testosterone produced, the boy with this disorder have an above average height, a smaller muscular mass, reduced pilosity with a gynoid disposition, wide hips, narrow shoulder, long legs and arms. Another common feature is gynecomastia (large mammary glands / breast enlargement in men) which occurs in almost 30% of patients. The adults with Klinefelter are usually taller than the men in their family.

Infertility is a common feature of this syndrome and is determined by hypogonadism (decreased testicular endocrine function). Microorchidism(small testes) occurs frequently. The men with Klinefelter's syndrome may have normal sex lives but they have azoospermia (no sperm is created ) in 95 to 98% of the cases or oligospermia (very low levels of sperm) in up to 5% of the cases.

The low amounts of testosteron also affect the bone metabolism, leading to weaker bones, an increased risk of fractures and a high predisposition to osteoporosis. Additionally, they have an increased risk of developing serious diseases such as breast cancer, germ cell tumors, rheumatoid arthritis, autoimmune disorders and diabetes mellitus.

The social and psychological development of the teenagers with Kleinfelter's syndrome is affected by the slow development of verbal skills and their poor speech skills. In school they may have problems learning or engaging in sportive activities and have more problems with "fitting in" than other teenagers, and the may also have poor social skills. They have a higher risk of developing depression, substance abuse problems and anxiety disorders. Their emotional problems range from shyness and immaturity to anxiety and aggressivity, but most of them are more quiet, undemanding, more obedient and more helpful and less active and self confident than other boys of their age. These psychological features persist in their adult life. As adults they can have a life similar to the other men, they may have families and friends and have successful careers although they may have to make more efforts in order to performs tasks that require reading or writing.