It is uncertain whether the JAK2 V617F mutation increases the thrombotic risk in patients with essential thrombocythemia, and it is unknown whether inherited thrombophilia is an additive risk factor in mutated subjects. We studied 132 patients with essential thrombocythemia, 38 of them (29%) with a history of thrombosis. The JAK2 mutation was present in 83 (63%), and inherited thrombophilia in 7. The mutated patients <60 years had a relative risk (RR) for thrombosis at any time of 3.83 (95%CI 1.27-11.49) in comparison with wild-type patients; in those with both the mutation and thrombophilia the RR was 2.23 (95%CI 1.57-3.18) and 7.66 (95%CI 2.66-22.03) in comparison with mutated or wild-type patients without thrombophilia, respectively. During the follow-up, only the homozygotes for JAK2 V617F were more prone to thrombosis (RR 17.25, 95%CI 2.33-127.4). Among the patients >60 years, no increase in RR was associated with the JAK2 mutation. In conclusion, in the younger patients with ET the thrombotic risk is higher in the JAK2 V617F-mutated and is further increased by the presence of inherited thrombophilia.