Oxygen-derived free radical generating capacity of polymorphonuclear cells in 27 patients with ulcerative colitis, 10 with acute bacterial diarrhea and 20 healthy volunteers, was measured by the luminol-dependent chemiluminescence method by stimulation of formyl-methionyl-leucyl-phenylalanine. Oxidative free radical generating capacity of polymorphonuclear cells in patients with active ulcerative colitis was markedly enhanced as compared with control (p less than 0.01), while this enhanced free radical production by the cells was not detected at remission stage. Serial analyses revealed that oxidative free radical production by the cells in patients with ulcerative colitis was markedly enhanced with clinical deterioration of the disease, but it returned to normal level with an improvement of clinical features. These results suggest that the increased oxidative free radical production by polymorphonuclear cells could be related to the pathogenesis or aggravation of ulcerative colitis.