There are suggestions of an anticancerogenic effect of allium vegetables and their associated organosulfur components against several cancer types, including gastric cancer, but the issue remains open to discussion and quantification. The present critical review discussed the history, the health properties, the chemistry, the anticancerogenic evidences from experimental studies, and the anticancer mechanisms of allium vegetables. We also summarized findings from epidemiological studies concerning the association between different types of allium vegetables and gastric cancer risk, published up to date. Available data, derived mainly from case-control studies, suggested a favorable role of high intakes of allium vegetables, mainly garlic and onion, in the etiology of gastric cancer. In particular, of 10 studies, 7 suggested a favorable role of high intake of total allium vegetables and gastric cancer. All 14 studies on garlic and most studies on onion (more than 80%) reported a beneficial role of these allium types against gastric cancer. However several limitations, including possible publication bias and the difficulty to establish a dose-risk relationship, suggest caution in the interpretation. Evidences on other types of allium vegetables, as well as on the influence of different gastric cancer anatomical and histological types, are less consistent.