It has been suggested that polycythemia vera (PV) could be preceded by an "early" phase of the disease (e-PV), in which the increase in the red cell parameters is lower than required for a PV diagnosis. In this study, we compared the clinicopathologic and molecular features of 17 patients with e-PV with those of 14 patients with essential thrombocythemia (ET) and 19 with PV. The results for e-PV were more similar to those for PV than for ET. In fact, patients with e-PV were characterized by an increase in the red cell parameters, splenomegaly (P<.05), and hepatomegaly (P=.038), together with hypercellular bone marrow due to increased erythropoiesis and granulopoiesis, associated with megakaryocytic hyperplasia, with pleomorphic aggregates (P<.001). The frequency of the JAK2V617F mutation was similar in e-PV (16 cases tested [100%]) and PV (18/19 [95%]) but was significantly lower (7/13 [54%]) in ET (P=.0007). We propose a diagnostic algorithm helpful to distinguish ET from the early prepolycythemic phase of PV.