We report on a 19-year-old girl with upper extremity deep vein thrombosis after catheter indwelling whose thrombophilic study disclosed the coexistence of factor V Leiden and the prothrombin G20210A mutation. The family study identified five other members who were also heterozygous for both mutations. This is the first case of upper extremity deep vein thrombosis with the co-inheritance of both genetic defects. It provides further evidence that thrombophilic defects mostly require additional triggering factors to induce a thrombotic event and suggests that in young patients with this venous thrombotic location, a thrombophilic search should be performed even when there are other acquired thrombotic risk factors.