The incidence of gastric cancer in the countries of South East Asia is variable, ranging from age-standardized rates of 20.9/105 (men) and 10.4/105 (women) in Hanoi, Vietnam to 4.1/105 (men) and 2.1/105 (women) in Khon Kaen, Thailand. The reasons for these differences are unknown. Possible explanations are differences in dietary habits, alcohol drinking, smoking and/or the prevalence of infection with Helicobacter pylori (H. pylori). A case-control study was conducted in Khon Kaen, Thailand, to study the role of these factors in gastric cancer carcinogenesis. 131 gastric cancer cases and 262 matched controls were recruited for the study. Information on dietary habits, alcohol drinking and smoking were collected by a structured questionnaire. Blood samples were available from 111 cases and 232 controls for H. pylori assay. Using an unconditional logistic regression model controlling for age and sex, we assessed the effects of dietary habits, alcohol drinking, smoking and H. pylori infection on the risk of gastric cancer. A high intake of salt (OR=1.8; 95%CI 1.1-3.0) and fermented foods (OR=1.9; 95%CI 1.1-3.3) was found to be associated with an increased risk. Preference for spicy food was not associated with gastric cancer risk in this population. Although there were negative associations between gastric cancer and vegetable and fruit intake, they were rather weak (OR 0.8 for both) and non significant. There were also weak (non-significant) associations with smoking and alcohol consumption, and no association with H. pylori infection (OR=0.6; 95%CI 0.4-1.0). Infection of H. pylori was associated with various indicators of crowding.