Homocysteine (Hcy) is an intermediary product in methionine metabolism and an elevation in plasma Hcy is a sensitive biomarker for an imbalance in the integrated pathways of one-carbon metabolism. More recently, there has been interest in the potential links between total Hcy, folate and cancer. In this study, the association of plasma Hcy levels with the breast cancer risk was investigated. Questionnaire information and blood samples were taken before treatment from 146 women with newly diagnosed, histologically confirmed breast cancer and 285 age-matched control women who were admitted for health examination. Plasma levels of Hcy and folate were measured by enzyme conversion immunoassay and radioassay, respectively. Dietary intake of B-group vitamins was estimated using a semi-quantitative dietary questionnaire. Logistic regression was used to calculate odds ratios (ORs) and their 95% confidence intervals (CIs). Elevated plasma Hcy levels were significantly linked to increased risk of breast cancer (adjusted OR = 2.89, 95% CI = 1.70-4.92 for the highest tertile as compared with the lowest tertile). Moreover, a similar pattern of enhanced breast cancer risk at higher plasma Hcy levels was observed in both pre-menopausal and post-menopausal women. And this consistent pattern did not differ substantially by level of dietary intake of B-group vitamins. The current study results seem to suggest a possibility that the plasma Hcy levels could be a metabolic risk factor for breast cancer. Future studies are needed to prove causality and provide insight on the mechanism of action of Hcy in breast tumorigenesis.