The activation of circulating polymorphonuclear leukocytes was determined in terms of superoxide radical generation and granulocyte elastase release in untreated patients with ulcerative colitis (N = 10) and Crohn's disease (N = 9) in remission and in control subjects (N = 10). Superoxide radical generation was determined by monitoring spectrophotometrically the reduction of ferricytochrome, after stimulation of cells with phorbol myristate acetate. Plasma elastase concentration was measured by a solid-phase enzyme immunoassay technique as the complex with alpha-1-proteinase inhibitor. Superoxide formation by polymorphonuclear leukocytes from patients with ulcerative colitis and Crohn's disease was significantly lower compared with controls [median (range) nmol/min/mg protein: Crohn's disease 7.8 (7.1-9.6); ulcerative colitis 8.25 (7.4-10.3); controls 14.7 (13.6-15.8)] (P < 0.001), while no difference was found between the two groups of patients. In contrast plasma elastase levels in patients with ulcerative colitis and Crohn's disease were similar to that of controls. This defective respiratory burst of polymorphonuclear leukocytes in patients with inflammatory bowel disease in remission, in absence of an altered degranulation, could represent an important factor for the pathogenesis of these diseases.