Allostatic load, the physiological accumulation of the effects of chronic stressors, has been associated with multiple adverse health outcomes. Flattened diurnal cortisol rhythmicity is one of the prototypes of allostatic load, and has been shown to predict shorter survival among women with metastatic breast cancer. The current study compared diurnal cortisol slope in 17 breast cancer patients and 31 controls, and tested associations with variables previously found to be related to cortisol regulation, i.e, abdominal adiposity, perceived stress, social support, and explicit memory. Women with metastatic breast cancer had significantly flatter diurnal cortisol rhythms than did healthy controls. Patients with greater disease severity showed higher mean cortisol levels, smaller waist circumference, and a tendency toward flatter diurnal cortisol rhythms. There were no relations between cortisol slope and psychological or cognitive functioning among patients. In contrast, controls with flatter rhythms showed the expected allostatic load profile of larger waist circumference, poorer performance on explicit memory tasks, lower perceived social support, and a tendency toward higher perceived stress. These findings suggest that the cortisol diurnal slope may have important but different correlates in healthy women versus those with breast cancer.