Objective The objective of this study is to assess the incidence and risk of mood disorders, including major depression, anxiety, and bipolar disorders, in Taiwanese patients after the diagnosis of breast cancer compared with a matched cohort. Methods From January 2000 to December 2005, 26,629 newly diagnosed breast cancer patients were enrolled by the Taiwan National Health Insurance program database. The control cohort was selected randomly from 1,000,000 National Health Insurance beneficiaries from a population of 21,400,826 enrolled throughout Taiwan. Each patient was matched with one subject without breast cancer by age, sex, and presence of comorbidities with the same diagnosis index date. The diagnosis of mood disorders was defined by compatible International Classification of Diseases, 9th revision, clinical modification codes plus the prescription of antidepressants for at least 30 days. Results The overall incidence rate ratio of mood disorders was 1.33 (95% CI 1.28–1.39, p < 0.001) in the breast cancer cohort compared with the matched cohort. The incidence rate ratios for specific mood disorders were 2.06 for bipolar disorder (95% CI 1.37–3.15 p = 0.0003), 1.94 for major depressive disorder (95% CI 1.76–2.13 p < 0.001), and 1.22 for anxiety (95% CI 1.16–1.27 p < 0.001). Independent risk factors for developing mood disorders included breast cancer, as well as age, hypertension, chronic obstructive pulmonary disease, autoimmune disease, ischemic heart disease, and cerebrovascular disease. Conclusions Breast cancer is a prominent risk factor for mood disorders, including major depressive disorder, anxiety, and bipolar disorder. The impact is most potent in the first year after diagnosis. Psychological support is a critical issue in these patients. Copyright © 2013 John Wiley & Sons, Ltd.