Women with breast cancer undergo treatments that decrease the chance of recurrence of cancer, but are associated with several side effects, including declines in memory and attention and other thinking abilities. The causes of these declines are not known. However, we know that (i) people with cancer may have high levels of molecules in the blood (cytokines) that reflect inflammation; (ii) injection of cytokines into animals, and their use to treat some human diseases, can lead to decreased memory and attention; and (iii) in some advanced cancers cytokines predict disease outcome. This longitudinal study evaluates the relation of cytokines to decreased thinking abilities and to disease outcome over time. Results of this study may help develop interventions to prevent or minimize cognitive decline and identify women who are at high risk for recurrence, and such information could be used in treatment decisions and in the development of new treatment options.