High-arousal negative affect, self-reported MVPA, area-under-the-curve of diurnal cortisol secretion, and goal adjustment capacities were assessed in a longitudinal sample of 145 female breast cancer survivors. Based on hierarchical linear modeling, breast cancer survivors reported increased levels of both MVPA and cortisol secretion if they experienced higher (as compared with lower) levels of high-arousal negative affect than their personal average. Furthermore, within-person negative affect was associated with: (a) higher MVPA among participants with high (but not low) goal reengagement capacities; and (b) elevated cortisol secretion among participants with low (but not high) goal reengagement capacities. High-arousal negative affect may exert differing functions among breast cancer survivors in that it can trigger adaptive health behaviors, yet simultaneously elevate diurnal cortisol secretion. In addition, being able to engage in new goals may be a necessary condition for breast cancer survivors to experience the beneficial behavioral effects of high-arousal negative affect, and it may prevent the adverse effect of negative affect on enhanced cortisol output. (PsycINFO Database Record (c) 2019 APA, all rights reserved).