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.