We describe here the rare coexistence, at the time of diagnosis, of a myeloproliferative neoplasm (MPN) and non-Hodgkin's lymphoma (NHL) in a 74-year-old patient who presented with thrombocytosis and signs of portal hypertension on physical examination. Abdominal computed tomography scan demonstrated extensive portal vein system thrombosis. Secondary causes of thrombocytosis were excluded. JAK2 V617F mutation was present in the peripheral blood, while bone marrow biopsy revealed marginal zone B-cell lymphoma. Molecular analysis failed to detect BCR-ABL rearrangement in peripheral blood cells. Simultaneous occurrence of MPN and NHL was diagnosed. This case may be of interest not only due to the rare coexistence of PMN and NHL, but also because of the undetermined clinical significance of JAK2 mutation in this subset of patients.