The V617F JAK2 mutation reported in Ph-negative myeloproliferative diseases (MPDs) induces the constitutive activation of JAK2, which produces an increased phosphorylation of signal transducer activator of transcription (STAT). In this study, we have analyzed a series of 114 patients (54 with polycythemia vera [PV], 44 with essential thrombocythemia [ET], 12 with idiopathic myelofibrosis [IM], and 4 with myelofibrosis secondary to MPD) for the expression pattern of phosphorylated STAT-3 and STAT-5 (pSTAT-3 and pSTAT-5, respectively) by immunostaining bone marrow biopsies. We found 3 specific patterns of pSTAT-3 and pSTAT-5 expression, significantly different from the normal staining pattern: uniformly increased pSTAT-3 and pSTAT-5 expression in PV, increased pSTAT-3 and reduced pSTAT-5 expression in ET, and uniformly reduced pSTAT-3 and pSTAT-5 expression in IM. A moderate increase of pSTAT-3 and pSTAT-5 expression was observed in secondary forms of erythrocytosis and thrombocytosis. In all evaluated MPDs, the pSTAT-5 and pSTAT-3 expression pattern was not influenced by the presence of V617F JAK2 mutation. These findings underline the importance of bone marrow histology in the differential diagnosis of Ph-negative MPD and support the hypothesis that V617F mutation simply contributes with other molecular defects in allowing the PV, ET, or IM phenotype to emerge.