The superoxide released from neutrophils works as a bacteriocidal agent in the biological protection system. On the other hand, superoxide also has been known to progress inflam-mation, tissue damage and fibrisis. Therfore, the superoxide production by neutrophils of patients with diffuse panbronchiolitis (DPB), idiopathic interstitial pneumonia (IIP) and collagen vascular disease with interstitial pneumonia (CVD+IP) were studied in relation to disease activity and progression. Superoxide production was measured by deduction of cytochrome C with a spectrophotometer. The superoxide production was higher in DPB than in the normal controls. The superoxide in DPB showed a positive correlation with CRP and white blood cell count, but inverse correlation with arterial blood oxygen pressure. High levels of superoxide in DPB decreased following erythromycin therapy with clinical improvement. The averagr levels of superoxide in the patients with IIP and CVD+IP were lower than those in the normal controls. However, the superoxide level in the patients with IIP and CVD+IP in the active stage was higher than that in those in the nonactive stage. In the active stage, the increase of steroid dose was follwed by a decrease of superoxide production in IIP and CVD+IP. These findings indicate that measurement of superoxide production by neutrophils could be an useful means to evaluate activity in diffuse pulmonary diseases.