Background: Vitamin B12 deficiency is associated with impaired DNA synthesis, repair and methylation. Sufficient intake of vitamin b12 has been hypothesised to lower breast cancer risk. So we conducted this case–control study to assess role of vitamin b12 in breast cancer in postmenopausal women in Delhi, India. Methods: We included 60 postmenopausal women diagnosed with breast cancer and 60 age matched postmenopausal healthy women as controls. Serum vitamin b12 levels were estimated using chemiluminescence based assay on access 2 (Beckman Coulter systems). Statistical analysis was done using SPSS (IBM, USA). Results: Serum vitamin b12 levels were found to be significantly lower in breast cancer patients as compared to healthy controls (p value < 0.05). The levels were found to be significantly decreased in patients with ER–/PR– receptor status. Conclusion: Vitamin B12 may have a role in carcinogenesis and may affect the expression of estrogen/ progesterone receptor in breast cancer.