Background/Aims: Helicobacter pylori (Hp) plays an important role in the formation of gastric and duodenal ulcers in a syndrome characterized by a high urease activity. On the other hand, enhancement in oxygen radical production is observed in the patients with peptic ulcers. The present study was designed to investigate the relationship between the different aspects of gastric mucosal injury, urease activity, oxygen free radical production in gastric specimens. Methods: Biopsy specimens were obtained from 32 patients (7 normal control; 18 gastritis; 7 gastric or duodenal ulcers). Urease activity was detected by a rapid urease test (CLO test). Oxygen free radical production was measured using luminol-dependent chemiluminescence (ChL) in the biopsy sample. Results: The CLO-positive rates were 100% in patients with ulcer, and 44.4% in the patients with gastritis. On the other hand, it was 0% in controls. ChL value was significantly increased in patients with ulcer and gastritis, especially in CLO-positive ulcer specimens. Conclusions: The positive rates of CLO test and elevated ChL value are strong indications for the existence of Hp, as well as for an enhanced oxygen free radical producing ability of infiltrated neutrophils. (Kor J Gastroenterol 2000;35:163 - 169)