Approximately 30% of breast cancer survivors report persistent fatigue of unknown origin. We have previously shown that cancer-related fatigue is associated with alterations in immunological parameters and serum cortisol levels in breast cancer survivors. The current study examined the diurnal rhythm of salivary cortisol in fatigued and non-fatigued breast cancer survivors. Salivary cortisol measures were obtained from breast cancer survivors with persistent fatigue (n=13) and a control group of non-fatigued survivors (n=16). Participants collected saliva samples upon awakening and at 1200, 1700, and 2200 h on two consecutive days. Diurnal cortisol slope for each day was determined by linear regression of log-transformed cortisol values on collection time and analyzed using multi-level modeling. Fatigued breast cancer survivors had a significantly flatter cortisol slope than non-fatigued survivors, with a less rapid decline in cortisol levels in the evening hours. At the individual patient level, survivors who reported the highest levels of fatigue also had the flattest cortisol slopes. Group differences remained significant in analyses controlling for demographic and medical factors, daily health behaviors, and other potential confounds (e.g. depressed mood, body mass index). Results suggest a subtle dysregulation in hypothalamic-pituitary-adrenal axis functioning in breast cancer survivors with persistent fatigue.