Low gastric juice total vitamin C concentration in the presence of Helicobacter pylori (H. pylori) infection probably plays a role in gastric carcinogenesis. In vitro vitamin C has been shown to inhibit the growth of H. pylori. The aims of this study were to determine the effect of high dose vitamin C administration on H. pylori infection and on gastric juice total vitamin C concentration in patients with H. pylori related chronic gastritis. Sixty patients with dyspeptic symptoms and proven chronic gastritis and H. pylori infection, who were undergoing routine endoscopy, entered the study after giving informed consent. They were randomly coded into two treatment groups. Group 1 (controls, n = 28) were treated with antacids for 4 weeks and Group 2 (n = 32) received vitamin C 5g daily also for 4 weeks. Nine patients did not complete the study and were excluded. Plasma and gastric juice total vitamin C levels were measured at baseline, at the end of 4 weeks treatment and again 4 weeks after treatment cessation. In the control group H. pylori infection remained unchanged in all 24 patients throughout as did the mean gastric juice total vitamin C concentration. However, in the vitamin C treated group eight of 27 patients (30%) who completed the treatment course the H. pylori infection was eradicated (P = 0.01). In these patients the mean gastric juice total vitamin C concentration rose significantly from 7.2 +/- 1.6 micrograms/ml after 4 weeks treatment (P < M 0.001) and 19.8 micrograms/ml 4 weeks after treatment was discontinued (P < 0.001). In the remaining 19 patients with persistent H. pylori infection, the mean gastric juice total vitamin C concentration rose less than in those with successful H. pylori eradication; 6.3 +/- 1.7 micrograms/ml before treatment, 10.8 +/- 1.5 micrograms/ml after 4 weeks treatment (P < 0.05) and a return to pre-treatment levels (7.1 +/- 2.7 micrograms/ml) 4 weeks after vitamin C intake stopped. There were no side effects of vitamin C treatment. This study has shown that 4 weeks daily high dose vitamin C treatment in H. pylori infected patients with chronic gastritis resulted in apparent H. pylori eradication in 30% of those treated. In those patients there was also a highly significant rise in gastric juice total vitamin C concentration which persisted for at least 4 weeks after the treatment ceased. A significant, though less marked, gastric juice total vitamin C concentration increase was observed during vitamin C treatment even in subjects with persistent H. pylori infection, though this was not maintained after treatment ended. The mechanism whereby vitamin C treatment appeared to result in H. pylori eradication is unclear. Further confirmatory studies are indicated.