A population-based case-control study is proposed to study the effect of Chinese green tea on risk of stomach cancer. Based on findings from recent experimental and epidemiologic studies it is hypothesized that the consumption of green tea decreases the risk of stomach cancer and the effect is stronger than that of the consumption of black tea. In light of recent findings on the beneficial effect of garlic and allium vegetables on stomach cancer risk, the study also proposes to examine whether these vegetables consumed raw, semi-cooked, or fully-cooked form would confer the same degree of protection. The study will be conducted at Hongko district and Nanhui county, two areas of Shanghai, China. All the incidence cases of primary stomach cancer occurring over a two-year period are eligible. The community controls will be one-for-one matched to the cases on sex, age, and area of residence. With the high incidence rate in this study population, 800 cases and 800 controls are expected to be enrolled. Home interview will be conducted to obtain information on type and frequency of tea consumption, type and quantity of allium vegetables consumed separately in raw, slightly cooked, or fully-cooked categories, dietary intake in four different time periods corresponding to historical events in China, education and occupation, tobacco and alcohol use, past medical history and family medical history. Standard analytic methods for matched case-control studies will be performed. Stomach cancer is prevalent in many parts of the world. If green tea is found to protective, it would add to the limited list of agents which have anticarcinogenic effect. Information on the mode of allium vegetables intake and the stomach cancer risk will be relevant for prevention application.