Background: Splanchnic vein thrombosis (SVT) is a severe complication of essential thrombocythemia (ET). No clear explanation has been given for the occurrence of thrombosis in this unusual site in patients with ET, but the existence of a specific association between unexplained SVT and the JAK2 mutation has been reported. Methods and Results: The present study describes SVT (portal and splenic vein thrombosis) in a young woman as the first presenting symptom of latent ET. Extensive screening for thrombophilia was negative. Our patient in fact did not fulfill the WHO diagnostic criteria for myeloproliferative disease (MPD), while she had splenomegaly and developed features suggestive of latent ET during follow-up. Conclusions: In these patients with SVT, the detection of JAK2V617F mutation is diagnostic for masked MPD as could be documented by bone marrow histopathology. The presence of JAK2V617F mutation should be considered per se a prothrombotic state for cerebral, coronary and peripheral microvascular disturbances and for SVT but not for deep vein thrombosis. Anticoagulation is the treatment of choice for all SVT and proper treatment of the MPD is recommended in patients with SVT associated with the JAK2V617F mutation.