ABSTRACT Women 65 and older (older women) account for nearly half of all new cases of breast cancer. With the graying of America the absolute number of older women diagnosed and undergoing breast cancer treatment will almost double by the year 2030. Treatment guidelines for these older patients include systemic therapy and older women are interested in chemotherapy for even small returns in survival extension. But systemic therapy is not without side effects, and numerous studies have documented cognitive decline after receipt of these agents. Imaging and animal studies confirm that cancer chemotherapy affects brain structure and function. However, very little is actually known about cognitive decline in older patients, because virtually all of the existing research has been conducted in younger patients. Since aging itself is associated with cognitive decline, older patients are likely to be particularly vulnerable to the adverse cognitive effects of systemic therapy. Our preliminary work suggests that this is the case, but this has never been empirically tested. This study will be the first large scale, prospective, controlled investigation to evaluate cognitive changes in older cancer patients. We use the vulnerability model of cancer survivorship to describe systemic therapy effects on cognition over a 12 month period, test associations between cognition and quality of life and to evaluate whether APOE polymorphisms moderate cognitive outcomes. We have assembled a team of oncologists, geriatricians, neurologists, neuro-, cognitive and behavioral psychologists and consumers from Lombardi Comprehensive Cancer Center, Memorial Sloan-Kettering Cancer Center, Boston University and Y- Me (a national consumer advocacy organization). We will enroll 325 newly diagnosed older breast cancer patients and an equal number of non-cancer friend controls. Participants will undergo baseline (pre-systemic therapy) neuropsychological testing and telephone interviews; clinical data will be abstracted from records. Participants will repeat cognitive testing and QOL measures 12 months after baseline. The primary outcome is change in the summary score on tests in the Attention, Working Memory, and Psychomotor Speed Domain. Four additional domains are included as secondary outcomes to assess broader cognitive function and examine differential impact: Language; Executive Functioning; Learning and Memory; Visuospatial. The results of this study will contribute to designing appropriate regimens for older women, developing preventive interventions, informing clinical decision-making about treatment, and guiding second generation studies. Overall, this topic has high research, clinical and public health importance, given the projected growth in the older population, rising incidence with advancing age, trends towards increasing use of systemic therapy in older patients, use of more aggressive dosing regimens, high survival rates, and increasing life expectancy.