Background and Objective: Polycythemia vera (PV) and essential thrombocytemia (ET) are chronic myeloproliferative diseases (MPD) characterized by overactive hemopoiesis. A single point mutation of JAK2 (Val617Phe) has been detected in PV, ET and myelofibrosis (MF). The aim of this work was to investigate the JAK2 mutation in patients with MPD and to compare the results to those of the endogenous formation of BFU-E erythroid colonies (EEC). Finally, different sources of hematopoietic cells to obtain DNA were evaluated., Patients and Method: In this work 146 patents were studied (81 MPD: 27 PV, 28 ET, 11 MF and 15 with myeloid chronic leukemia). Moreover, 28 patients showed secondary polycythemias or reactive thrombocytosis, 8 MPD/myelodysplastic syndromes and 29 other disorders. In 54 patients, EEC were also evaluated. Peripheral blood cells were used as source of DNA in 122 patients, bone marrow in 33, cells from BFU-E in 14 and cells from EEC in 24 patients. Their DNA samples were analyzed using an allele-specific polimerase chain reaction methodology., Results: The JAK2 mutation was present in 96% of PV patients, 59% of ET and 63.6% of MF. None of the remaining patients showed this mutation. Diagnostic agreement was excellent between EEC and the mutation (kappa index = 0.93; 97% positive agreement and 95% negative agreement). DNA was obtained in 119 out of 122 samples from peripheral blood, in all patients with bone marrow, and in 50% of patients with BFU-E or EEC. In 7 cases, samples from different cell sources were studied. Their results were identical., Conclusions: The V617F mutation of JAK2 is present in most of PV patients and half of those with MF or ET. There is an excellent concordance with the EEC results.