Respiratory burst of leukocytes was determined in Crohn's disease, intestinal Behçet's disease and ulcerative colitis by using chemiluminescence assay. Oxygen-derived free radicals of monocytes in Crohn's disease and those of polymorphonuclear leukocytes in intestinal Behçet's disease were significantly raised as compared with those of controls. In ulcerative colitis, respiratory burst activities of both polymorphonuclear leukocytes and monocytes were found to be highly associated with the disease activities. These results suggest that active oxygen species from leukocytes may participate in the formation of intestinal mucosal lesions in inflammatory bowel diseases via auto-oxidative process.