Neutrophil NADPH oxidase is a membrane-bound enzyme complex responsible for the reduction of oxygen to superoxide anion in the respiratory burst. Impaired neutrophil function has often been reported in myeloproliferative diseases and myelodysplastic syndromes (MDSs), but its laboratory features have not been well characterized. Traditionally, NADPH oxidase activity has been evaluated with a microscopic method by nitroblue tetrazolium salt reduction to blue-black insoluble formazan granules identified in positive neutrophils by microscopy. We investigated neutrophil NADPH oxidase in 22 patients with chronic myeloproliferative disorders (cMPDs) and 15 patients with MDSs, using the microscopic method as well as a photometric microplate assay, which monitored the cytochemical reaction for 30 min. The relationship between cMPD and patient susceptibility to infections was also investigated. In the photometric assay, the mean enzyme activity in MDSs and cMPDs was lower than in normal subjects. NADPH oxidase activity was greater in cMPDs (except myelofibrosis) than in MDSs (except chronic myelomonocytic leukemia), and these differences were less evident with microscopy. Moreover, for cMPDs, patients with susceptibility to infections showed a lower NADPH oxidase activity than patients without infections.