By using novel molecular markers, it is possible to gain information on both the classification and the pathophysiology of the chronic myeloproliferative neoplasia (MPN). In a group of 36 Mexican mestizo patients with MPN, we studied five molecular markers: The BCR/ABL1 fusion gene, the JAK2 V617F mutation, the JAK2 exon 12 mutations, the MPL W515L mutation and the MPL W515K mutation; 17 patients with essential thrombocythemia (ET), eight with polycythemia vera (PV), four with primary mielofibrosis (MF), five with undifferentiated MPN, one with primary erythrocytosis and one with familial thrombocytosis. Patients with the BCR/ABL1 fusion gene were excluded. Twelve individuals with the JAK2 V617F mutation were found; 11 of them had been clinically classified as PV and one had been classified as MF. One patient with the MPL W515L was identified with a clinical picture of ET. No individuals with either the MPL W515K mutation or the JAK2 exon 12 mutations were identified. Of the 17 individuals with ET, six (35%) had the JAK2 V617F mutation and one (6%) was found to have the MPL W515L mutation. Of the eight individuals with PV, five displayed the JAK2 V617F mutation, whereas of the four patients with MF, one had the JAK2 V617F mutation. The most consistent relationship was that between PV and the JAK2 V617F mutation (p=0.08). In the diagnosis and classification of the MPN, in addition to the newly identified molecular markers, clinical and laboratory data are still very important.