Venous thromboembolic disease was rarely described in association with Klinefelter's syndrome. It is nevertheless more frequent than in a genetically normal male population (5-20 times). This abnormality is not clearly explained. We report a classical case of Klinefelter's syndrome detected by the help of a venous thromboembolic disease. The patient was of blood-group A (the relative incidence of thrombosis is raised among group-A persons as compared with group-O persons). The evaluation disclosed a high rate of beta-thromboglobulin suggesting an underlying hypercoagulability. The abnormal incidence of venous thromboembolic disease among Klinefelter patients may find a partial explanation in this way.