The effect of oral vitamin C supplementation on intragastric ascorbate levels and gastric mucosal DNA damage as measured by the 32P-postlabelling assay was assessed in 43 patients. In patients with normal gastric mucosa, treatment with vitamin C resulted in elevation of intragastric ascorbate levels in all cases. In the presence of chronic atrophic gastritis, however, the effect was variable. Gastric mucosal DNA damage was decreased in 28 of the 43 patients after vitamin C supplementation (P = 0.01; Wilcoxon sign rank test). This supports epidemiological evidence that suggests vitamin C may exert a protective effect against the development of gastric cancer.