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Jan Samuelsson, MD, PhD, Department of Medicine, Section for Hematology, Karolinska Institute at Stockholm Söder Hospital, S-118 83 Stockholm (Sweden) We read with interest the report by Piva et al. [1] describing an impaired function of the NADPH oxidase, assessed as the reduction of nitroblue tetrazolium (NBT) after stimulation with phorbol myristate acetate (PMA), in patients with various stages of myelodysplastic syndromes and different chronic myeloproli-ferative disorders. In that paper, the authors concluded that PMAinduced NBT reduction was reduced in a few patients with polycythemia vera (PV) that were investigated. Since our group in a series of studies has come to different conclusions regarding the oxidative burst in polymorphonuclear neutrophil granulocytes (PMN) in PV, we would like to briefly comment on the part of the paper by Piva et al. [1] that concerned PV. A more detailed review of our findings has recently been published [2]. Prior to 1988, all studies of oxidative metabolism in PV PMN had been performed with latex beads as stimulus. Cooper et al. [3] reported an increased oxygen consumption and NBT reduction, whereas other studies showed normal [4] or reduced NBT reduction [5, 6]. In the article by Piva et al. [1], our first study [7] was quoted as showing that ‘in other myeloproliferative disorders, most authors have described normal or even activated neutrophils in PV. Though it is true that spontaneous as well as N-formyl-methionyl-leucyl-phenylalanine (fMLP)-in-duced adherence and chemotaxis was normal, our study demonstrated for the first time a stimulus-specific defect in the oxidative metabolism of PV PMN, assessed as chemiluminescence (CL), after stimulation with surface-receptor-dependent stimuli, i.e. fMLP and leukotriene B4 (LTB4). However, in contrast to the findings by Piva et al. [1], we found that CL induced by PMA was normal in PV PMN. Furthermore, in a later study [8], microscopic NBT reduction was similar in both unstimulated and PMA-stim-ulated PMN, but significantly reduced after fMLP stimulation. We subsequently showed that the generation of superoxide anions was significantly impaired in PV PMN after stimulation with fMLP, but once again, the response to PMA was normal [9]. Finally, we could show that CL induced by fMLP and PMA was normal in PMN from patients with chronic myelogenous leukemia (CML) and essential thrombocythemia (ET) [10], Moreover, studies of hydrogen peroxide generation (1⁄8C1⁄8) in single PMN and mono-cytes by flow cytometry revealed that resting and PMA stimulated 5⁄8C1⁄8 generation was normal in PV, CML and ET, as was fMLP induced 7⁄8C1⁄8 generation in ET. In contrast, fMLP induced 7⁄8 < 3⁄4