BACKGROUND: Psychological distress and coping related to a breast cancer diagnosis can profoundly affect psychological adjustment, possibly resulting in the disruption of circadian rest/activity and cortisol rhythms, which are prognostic for early mortality in metastatic colorectal and breast cancers, respectively. PURPOSE: This study aims to explore the relationships of cancer-specific distress and avoidant coping with rest/activity and cortisol rhythm disruption in the period between diagnosis and breast cancer surgery. METHODS: Fifty-seven presurgical breast cancer patients provided daily self-reports of cancer-specific distress and avoidant coping as well as actigraphic and salivary cortisol data. RESULTS: Distress and avoidant coping were related to rest/activity rhythm disruption (daytime sedentariness, inconsistent rhythms). Patients with disrupted rest/activity cycles had flattened diurnal cortisol rhythms. CONCLUSIONS: Maladaptive psychological responses to breast cancer diagnosis were associated with disruption of circadian rest/activity rhythms. Given that circadian cycles regulate tumor growth, we need greater understanding of possible psychosocial effects in cancer-related circadian disruption.