PURPOSE: The protective effect of Allium vegetables against carcinogenesis has been reported in experimental studies particularly focusing on the gut. Therefore, we conducted a hospital-based matched case-control study to explore the association between dietary Allium consumption and risk of breast cancer among Iranian women in northwest Iran. METHODS: A validated, quantitative, food frequency questionnaire was completed in 285 women (aged 25–65 years old) newly diagnosed with histopathologically confirmed breast cancer (grade II, III or clinical stage II, III) in Tabriz, northwest Iran, and the completed questionnaires were included in an age- and regional-matched hospital based-control study. The odds ratios (ORs) and 95% confidence intervals (95% CI) were estimated using conditional logistic regression models. RESULTS: Multivariate analysis showed that there was a negative association between the consumption of raw onion and risk of breast cancer after adjustment for covariates (OR, 0.63; 95% CI, 0.40–1.00); however, this association was insignificant. On the other hand, there was a positive association between consumption of cooked onion and risk of breast cancer, after adjustment for covariates (OR, 1.54; 95% CI, 1.02–2.32). However, reduced risk of breast cancer was associated with higher consumption of garlic and leek with adjusted ORs of 0.41 (95% CI, 0.20–0.83) and 0.28 (95% CI, 0.15–0.51), respectively. CONCLUSION: Our findings suggest that high consumption of certain Allium vegetables, in particular garlic and leek, may reduce the risk of breast cancer, while high consumption of cooked onion may be associated with an increased risk of breast cancer.