PURPOSE: To examine the utility of polycythemia rubra vera-1 (PRV-1)–specific reverse transcriptase polymerase chain reaction (RT-PCR) to discriminate essential thrombocythemia (ET) and polycythemia vera (PV) from secondary thrombocytosis (ST) or secondary erythrocytosis (SE). PATIENTS AND METHODS: We analyzed the expression of PRV-1 in granulocytes isolated from 37 patients with ET, 37 patients with PV, 25 patients with ST, 10 patients with SE, 25 patients with secondary leukocytosis (SL), five patients with chronic myelogenous leukemia (CML), five patients with chronic idiopathic myelofibrosis (IM), five patients with myelodysplastic syndrome (MDS), and 20 normal individuals by PRV-1–specific RT-PCR. In female patients, PRV-1 expression was correlated with clonality analysis as assessed by the human androgen receptor polymorphism assay. RESULTS: PRV-1 was not expressed in granulocytes isolated from normal individuals or from patients with ST, SE, CML, IM, MDS, and inflammatory/infectious SL. On the contrary, all ET patients, 35 of 37 PV patients, and five patients with acute postsurgery and posttraumatic SL overexpressed PRV-1. All the cases with monoclonal hematopoiesis (17 of 21 with ET and 12 of 12 with PV) expressed PRV-1, yet PRV-1 overexpression extended also over the cases of ET showing polyclonal hematopoiesis (four of 20). CONCLUSION: The overexpression of PRV-1 seems to be a useful tool for discriminating ET and PV from ST and SE, thus offering an innovative diagnostic approach on the basis of the detection of positive diagnostic criteria instead of exclusion criteria.