Background/Aims: Helicobacter pylori has been recognized as a important risk factor for gastric cancer, but its precise role in gastric carcinogenesis is not known. Accumulated evidence suggest that high dietary vitamin C reduces gastric cancer risk. The aim of this study was to assess the relationship between H. pylori infection and vitamin C concentraion in gastric juice. Another objective is to determine whether eradication of H. pylori affect the concentraion of vitamin C in gastric juice. Methods: Vitamin C concentration in gastric juice and plasma were measured by high performance liquid chromatography in 50 subjects without ulcer or cancer. In 20 H. pylori positive patients, eradication was attempted with amoxicillin 1000 rng, omeprazole 20 mg and clarithromycin 500 mg two times a day for 10 days. Results: There were 38 H. pylori positive and 12 negative patients. Total vitamin C and ascorbic acid concentraion of gastric juice in H. pylori positive patients were significantly lower than that in H. pylori negative patients(1.23mg/dL vs 2.22 mg/dL, 0.49mg/dL vs 0.86mg/dL). Eradication was achieved in 16(80%) of 20 H. pylori positive patients. In these 16 patients, gastric juice total vitamin C and ascorbic acid concentraion and the ratio of vitamin C in gastric juice to plasma rose and the dehydroascorbic acid proportion of total vitamin C in gastric juice reduced after treatment. But, in 4 patients in whom H. pylori infection persisted, no significant changes were observed in vitamin C concentraion of the gastric juice or plasma. Total vitamin C and ascorbic acid concentraion in gastric juice was influenced by gastric pH with significantly low values in patients with high gastric juice pH(pH >4). Conclusions: This study confirmed that H. pylori infection reduce vitamin C concentraion in gastric juice and successful eradication of H. pylori improves it. (Korean J Gastroenterol 1997; 30:18 - 29)