Objective To study the risk factors for contralateral breast cancer (CBC) in women after regular treatment of the primary breast cancer. Methods Between January 1997 to December 2002, the clinical data of 340 breast cancer patients at our institution were retrospectively analyzed. In all the patients a detailed analysis was carried out with respect to age, operation type, radiation therapy technique and dose, the use of chemotherapy or hormone therapy, and other clinicopathologic characteristics. The KaplanMeier method was used to estimate the actuarial rate of CBC. The Cox proportional hazard regression model was used to estimate the relative risk factors of CBC. Results Fourteen cases were diagnosed to be CBC, thus overall incidence of CBC was 4.1%. Ten-year CBC incidence (2.7%) was higher than 5-year incidence of CBC (1.4%). Univariate analysis showed that the risk factors of CBC at 5-year and 10-year included: ≤45 years old, medullary carcinoma, family history of breast cancer and being taken without endocrine therapy (P＜0.05), while chemotherapy and radiotherapy were not risk factors of CBC (P＞0.05). Mutivariate analysis showed that ≤ 45 years old and being internal breast radiotherapy were independent risk factors of CBC at 5-year and 10-year (P＜0.05). Conclusions CBC may occur in these primary breast cancer patients with age ≤45 years old, medullary carcinoma, family history of breast cancer. In order to reduce the incidence of CBC, endocrine therapy rather than internal breast radiotherapy should be performed in early breast cancer patients.