We investigated active oxygen (H2O2) production by neutrophils in patients with inflammatory diseases such as connective tissue disease and respiratory tract infection. The analysis was done by flow cytometry using as small volume of whole-blood as 100 microliters. H2O2 production as well as C-reactive protein (CRP) were increased in connective tissue diseases, and was decreased after steroid therapy. In a case of neuro-Behçet's disease, changes in H2O2 production was observed in agreement with those in clinical symptoms such as gait disturbance. H2O2 production as well as CRP and neutrophil count were increased in acute respiratory tract infection, but in chronic patients H2O2 production alone was rather reduced. In stages of enhanced H2O2 production, tissue damage or inflammation may exist, the degree of which seems to be well reflected by H2O2 production. During stages of impaired H2O2 production, care should be taken to prevent infections. Using flow cytometry, H2O2 production can be easily determined as one of functions of neutrophils.