BACKGROUND Stomach cancer is generally thought to evolve through a series of gastric mucosal changes, but the determinants of the precancerous lesions are not well understood. OBJECTIVE Our purpose was to assess risk factors for intestinal metaplasia and gastric dysplasia arising from chronic atrophic gastritis in a general population at high risk for stomach cancer. METHODS A population-based gastroscopic screening of more than 3000 residents was conducted in a county in China with one of the world's highest rates of stomach cancer. Information on the lifestyle and other characteristics of the participants was obtained by interview, and responses were compared between those in whom the most advanced gastric lesion was dysplasia or intestinal metaplasia versus those with chronic atrophic gastritis. RESULTS Cigarette smoking was found to nearly double the risk of transition to dysplasia and to be a mild risk factor for intestinal metaplasia. Smoking accounted almost entirely for the 55% higher prevalence of dysplasia among men than among women. Risk of transition to dysplasia had a weak association with several dietary factors and was increased among those participants with a family history of stomach cancer and with blood type A. CONCLUSIONS The findings provide strong evidence for a role of tobacco consumption and offer clues to other environmental and genetic factors involved in the process of gastric carcinogenesis.