Venous thromboembolism (VTE) includes deep vein thrombosis and its complications and pulmonary embolism. Cancer, surgery, prolonged immobilization, fractures, paralysis, use of oral contraceptives, and hereditary coagulopathies are classic risk factors for VTE. An increased incidence of VTE has been reported in patients with Klinefelter syndrome (KS), with a reported prevalence of 0.1-0.2% in the general population and up to 3.1% in infertile men. Despite the high rate of thromboembolic disease in patients with KS, the etiology of this phenomenon is not well understood, and most of our current knowledge is limited to small sample studies. We present the case of a 56-year-old man admitted for the management of a pulmonary embolism in whom a KS was accidentally discovered.