The Jak2(V617F) mutation is found in most classical BCR/ABL-negative myeloproliferative neoplasms (MPNs). Usually, heterozygosity of the mutation is associated with essential thrombocythemia (ET) and homozygosity with polycythemia vera (PV). Retrovirally transduced or transgenic animal models have shown that the mutation is sufficient for MPN development but that the level of expression is crucial for MPN phenotypes. Therefore we investigated the effect of an endogenous heterozygous expression of Jak2(V617F) in knock-in (KI) mice. These animals displayed constitutive JAK2 activation and autonomous erythroid progenitor cell growth. Mice suffered from marked polycythemia, granulocytosis and thrombocytosis. Spleens and marrows displayed myeloid trilineage hyperplasia. Most animals survived to develop advanced fibrosis in these organs at around 9 months of age. In conclusion, constitutive heterozygous expression of JAK2(V617F) in mice is not embryo-lethal but results in severe PV-like disease with secondary myelofibrosis and not in ET-like disease as expected from patient study.