Treatment of elderly patients with breast cancer is limited by the lack of evidence-based medicine due to exclusion of elderly persons from clinical studies and the difficulty of decision-making in an elderly population comprising subjects with heterogeneous health backgrounds. Individual variation of health conditions is larger in elderly patients than in young people. Since the risk-benefit balance of treatment depends on the health background, judgment of the vulnerability of individual patients is required in treatment planning. The comprehensive geriatric assessment (CGA) is a tool for acquisition of information for design of a cancer treatment plan based on evaluation of the vulnerability of elderly patients from various perspectives. The domains of the CGA include physical function, comorbidity, cognitive function, psychological status, social support system, nutrition, and medication. The CGA can be used to predict survival and adverse events; to identify health problems that cannot be detected based on medical history or examination; to recognize new problems that emerge during course observation; and to improve psychological conditions. However, application of the CGA in routine clinical practice requires establishment of a consensus regarding the survey method, evaluation scale, and use of results in decision-making. A prospective investigation of the CGA as an outcome index in elderly breast cancer patients is required to address these issues.