Studies have showed an association between type 2 diabetes and breast cancer in Western countries. The association should be confirmed in an Asian population which has a lower incidence of breast cancer. Our study aimed to compare the clinicopathologic characteristics of breast cancers in women with and without type 2 diabetes mellitus in China. Each group included 143 cases, similar in parity, body mass index, family history, mode of diagnosis, menarche age and hormone receptors except for progesterone receptor (PR). The diabetic patients were older, with a mean age of 58.3±10 years and the percentage of postmenopausal patients was 52%, which was higher than non-diabetic patients. Significant differences were found in tumor stage, the amounts of lymph node and with metastasis, these persisting after adjustment for age. Furthermore, a positive association with higher TNM status and PR negative rate was noticed in premenopausal but not postmenopausal diabetic patients. Our results indicate that type 2 diabetes mellitus is a negative prognostic factor for breast cancer, especially in premenopausal women.