The G20210A mutation in the prothrombin gene is associated with an increased risk of a first venous thromboembolic episode; few data are available about the long-term risk for recurrent venous thromboembolism and it is not known whether or not carriers of the mutation should be recommended lifelong anticoagulant treatment after the first thrombosis. We investigated 624 patients, referred for previous objectively documented deep venous thrombosis of the legs or pulmonary embolism, to determine the risk of recurrent thromboembolism in heterozygous carriers of the G20210A mutation in the prothrombin gene after the first episode of venous thromboembolism. After exclusion of other inherited (anti-thrombin, protein C, protein S deficiency and factor V Leiden) or acquired (anti-phospholipid antibody syndrome) causes of thrombophilia, 52 heterozygous carriers of the prothrombin mutation were compared with 283 patients with normal genotype. The relative risk for recurrent venous thromboembolism was calculated between groups using a Cox's proportional hazard model. The patients with the prothrombin mutation had a risk for spontaneous recurrent venous thromboembolism similar to that of patients with normal genotype (hazard ratio 1.3; 95% CI, 0.7-2.3). The circumstances of the first event (spontaneous or secondary) did not produce any substantial variation in the risk for recurrence. In conclusion, the carriers of the prothrombin mutation should be treated with oral anticoagulants after a first deep venous thrombosis for a similar length of time as patients with a normal genotype.