Klinefelter's syndrome is the most common cause of primary testicular failure. Previous reports have associated Klinefelter's syndrome with increased risk of thrombosis. The exact cause for this association is unknown, but hypoandrogenism affecting fibrinolysis has been implicated. The authors described a unique patient with Klinefelter's syndrome who presented with deep vein thrombosis of the leg and underlying mutations of MTHFR gene, increased factor VIII coagulant activity and an elevated anticardiolipin antibody. To the authors' knowledge, this combination of hypercoagulability risk factors in such a patient has not been previously reported. The authors also reviewed previously published reports of similar patients and discuss potential genetic mutations that may in part predispose this group of patients to venous thrombosis.