A new genetic anomaly predisposing to venous thrombosis was described in 1996, namely the transition of guanine (G) to adenine (A) at position 20210 in the 3-untranslated region of the prothrombin gene. This mutation is associated with high levels of plasma prothrombin and increased risk of thrombotic events in the venous system. We report the case of a man who, lacking known risk factors for thrombosis, suffered a massive pulmonary embolism and deep venous thrombosis in both lower legs. Thrombophilic analysis confirmed that the patient and close relatives were carriers of the heterozygotic 20210G/A variant of the prothrombin gene. Two relatives with the genetic defect had also suffered some type of deep venous thrombosis.