Klinefelter syndrome is the most common sex chromosome disorder and a common cause of primary hypogonadism. This congenital syndrome is seen in males with at least 1 additional X chromosome. While the majority of cases are caused by the 47,XXY karyotype, other karyotypes have also been identified. The presence of additional X chromosome(s) is responsible for testicular dysfunction and other clinical features, including signs and symptoms of androgen deficiency, increased height, and psychosocial abnormality. Phenotypical expression is variable, and the condition is thought to be underdiagnosed with only 40% of cases reported. Of note, Klinefelter syndrome is associated with increased mortality and morbidity from a variety of co-morbidities.