Portal vein thrombosis is, in some cases, related to a myeloproliferative disorder but the diagnosis of the latent forms may be difficult in case of normal blood counts. We report two cases of patient with portal vein thrombosis of unknown origin in whom the presence of the V617F mutation of the Janus Kinase 2 gene lead to the diagnosis of primary myeloproliferative disorder, confirmed on bone marrow examination. The search of the V617F mutation of the Janus Kinase 2 gene has to be performed in all cases of portal vein thrombosis of unknown origin.