XXY syndrome or 47, XXY condition is also known as Klinefelter's syndrome is a condition in which males possess extra X sex chromosome in their genotype. Usually females have an XX chromosomal makeup, and males an XY, affected individuals have at least two X chromosomes and at least one Y chromosome. Klinefelter's syndrome is the commonest sex chromosome disorder.  The condition is second most common condition caused by the presence of extra chromosomes. More than 0.1% of males are affected by the disease.

The common signs and symptoms are development of small testicles and reduced fertility. Patients may have many behavioral and physical abnormalities like infertility, difficulty in language learning and neuropsychological testing reveals deficits in executive functions. Other symptoms in adult may include youthful build, rounded body type, gynaecomastia and a lanky facial appearance. Patients will have hypogonadism which means they will have low endocrine functions while the testicles and penis will be normal in size and appearance. Hormonal profile may show low serum testosterone level but high serum follicle-stimulating hormone or FSH and luteinizing hormone or LH levels.

The patients will have increased risk of germ cell tumors, breast cancer, and osteoporosis. Some patients may have associated pulmonary disease, varicose veins, diabetes mellitus, and rheumatoid arthritis. The reliable method of identifying whether a patient is 47 XXY is by karyotyping only. In this procedure, a small blood sample is drawn. White blood cells are then separated from the sample, mixed with tissue culture medium, incubated, and checked for chromosomal abnormalities, such as an extra X chromosome. Prenatally the condition is diagnosed by chorionic villus sampling or amniocentesis which includes extracting of fetal tissues and the DNA is examined for the genetic abnormalities.

Testosterone treatment is advised to the patients who are in need of the more masculine appearance and identity. Patients may develop anxiety disorders and depression due to the hypogonadism and gynaecomastia.