In the fall of 1995, 3411 subjects in 13 rural villages in Linqu County, Shandong Province, China, began participating in a blinded, randomized 23 factorial trial to determine whether interventions can reduce the prevalence of dysplasia and other precancerous gastric lesions. One intervention is treatment for infection by Helicobacter pylori with amoxicillin and omeprazole. A second is dietary supplementation with capsules containing vitamin C, vitamin E, and selenium. A third is dietary supplementation with capsules containing steam-distilled garlic oil and Kyolic aged garlic extract. Investigators will evaluate histopathologic endpoints after gastroscopies with biopsies from seven standard sites in 1999. Initial data from pill counts and sampled blood levels of vitamin E, vitamin C, and S-allylcysteine indicate excellent compliance. Subjects have tolerated all interventions well, although 3.1% of those assigned to amoxicillin and omeprazole developed rashes, compared to 0.3% to those in the control group. Preliminary breath tests demonstrate substantial reductions in gastric urease activity, an indication of infection by Helicobacter pylori, among those assigned to amoxicillin and omeprazole.