In human studies, low vitamin C intake has been associated with more severe Helicobacter pylori gastritis and a higher incidence of gastric cancer. However, vitamin C supplementation has not been definitively shown to protect against gastric cancer. Using vitamin C-deficient B6.129P2-Gulotm1Umc/mmcd (gulo−/−) mice lacking L-gulono-γ-lactone oxidase, we compared gastric lesions and Th1 immune responses in H. pylori-infected gulo−/− mice supplemented with low (33 mg/L) or high (3,300 mg/L) vitamin C in drinking water for 16 or 32 weeks. Vitamin C levels in plasma and gastric tissue correlated with the vitamin C supplementation levels in gulo−/− mice. H. pylori infection resulted in comparable gastritis and premalignant lesions in wildtype C57BL/6 and gulo−/− mice supplemented with high vitamin C, but lesions were less severe in gulo−/− mice supplemented with low vitamin C at 32 weeks post infection. The reduced gastric lesions in infected gulo−/− mice supplemented with low vitamin C correlated with reduced Th1-associated IgG2c, gastric IFN-γ and TNF-α mRNA and higher H. pylori colonization levels. These results in the H. pylori-infected gulo−/− mouse model suggest that although supplementation with a high level of vitamin C achieved physiologically normal vitamin C levels in plasma and gastric tissue, this dose of vitamin C did not protect gulo−/− mice from H. pylori-induced premalignant gastric lesions. In addition, less severe gastric lesions in H.pylori infected gulo−/− mice supplemented with low vitamin C correlated with an attenuated Th1 inflammatory response. © 2007 Wiley-Liss, Inc.