The first international meeting on V617F JAK2 mutation in myeloproliferative disorders (MPD) was held by the PV-Nord group on behalf of the French Society of Hematology and Paris 13 University on November 18, 2005, in Paris (France). Twelve speakers, including representatives of the three European groups who discovered the V617F JAK2 mutation and international experts in the field of Philadelphia-negative MPD, presented original biological and clinical data that allow better insight in the relevance of V617F JAK2 mutation in the pathogenesis and management of those diseases. The role of V617F JAK2 in cytokine receptors trafficking and signaling was described. Follow-up of transgenic mice expressing V617F JAK2 showed that they develop typical features of myelofibrosis. Comparisons of JAK2 mutational status to clonality of hematopoiesis in essential thrombocythemia on the one hand, and to activation of transcription factors in myelofibrosis with myeloid metaplasia on the other hand, suggest that JAK2 mutation could be a second genetic event in a subset of patients. Alternatively, other gene mutation(s) have to be found to explain the development of V617F-negative MPD. In large series of MPD patients presented, clinical characteristics of mutated and non-mutated patients were found different. Finally, the place of V617F JAK2 testing in the diagnosis and management of MPD was discussed.