Klinefelter syndrome occurs in approximately 1/700 males, and it is widely undiagnosed. Venous thromboembolism is common in Klinefelter syndrome. In boys or childless men with unprovoked venous thromboembolism, Klinefelter syndrome is an important diagnosis to consider. A thorough somatic examination including the testes is important to raise suspicion of Klinefelter syndrome. This case report describes a man who is diagnosed with Klinefelter syndrome after having a pulmonary embolism. Since there were not any other factors that could explain his venous thromboembolism, his genetic condition was considered to be the underlying cause.