OBJECTIVES: To estimate prevalence of clinically diagnosed mental disorders among hospitalized breast cancer patients and demonstrate their demographic characteristics.   METHODS: The Nationwide Inpatients Sample (NIS) in the US between 2007 and 2011 was used for this study. Breast cancer patients were identified as ICD-9-CM codes (174.0–174.9), metastatic stage of breast cancer patients were identified as the ICD-9-CM code (196.0), and non-metastatic stage of patients were identified as ICD-9 code (2330). Prevalence of mental disorder was calculated as the proportion of any diagnosis of depression, depressive disorder, anxiety disorder, acute reaction to stress, and posttraumatic stress disorder (PTSD) among hospitalized breast cancer patients.    RESULTS: Among hospitalized breast cancer patients, prevalence of mental disorder was 6.1%(2007), 7.0%(2008), 7.4%(2009), 8.3%(2010), 9.3%(2011).  Patients were more likely (78%) to be diagnosed mental disorders between 30 and 60 years old. Among patients with any mental disorders, Caucasians had a higher proportion (77.3%) compared with African Americans, Hispanics and other ethnical groups. Patients with mental disorders had a higher proportion of Medicare beneficiaries, following private insurance, Medicaid and self-pay. CONCLUSIONS: Recognizing the increase of mental disorders among hospitalized breast cancer patients is important. Research is needed to ascertain how co-existing mental health disorders would affect the quality of  care and/or treatment strategies for hospitalized breast cancer patients.