[35-year old patient with severe thromboembolism].

We report a case of a 35 year old male with severe deep vein thrombosis of the lower limb on both sides and pulmonary embolism. A Klinefelter's mosaic (47,XXY [81%]/48,XXXY [19%]) was diagnosed. Because no other cause for this thromboembolism was found, we assume that in part, it was caused by the Klinefelter's mosaic. In all male patients presenting with thromboembolism, especially those with an unusual habitus, a Klinefelter's syndrome should be considered as differential diagnosis. Testosterone substitution therapy should be started in all patients with Klinefelter's syndrome to prevent further disease.