Background: The relationship between pre-surgical distress and diurnal cortisol following surgery has not been investigated prospectively in caregivers of coronary artery bypass graft (CABG) patients before. We aimed to examine the relationship between pre-surgical anxiety and caregiver burden and diurnal cortisol measured 2 months after the surgery in the caregivers of CABG patients. Method: We used a sample of 103 caregivers of elective CABG patients that were assessed 28.86 days before and 60.94 days after patients’ surgery. Anxiety and caregiver burden were assessed using the anxiety subscale of the Hospital Anxiety and Depression Scale and the Oberst Burden Scale respectively. Saliva samples were collected to measure cortisol area under the curve with respect to ground (AUCg) and diurnal cortisol slope. Anxiety and caregiver burden were entered into linear regression models simultaneously. Results: While high levels of pre-surgical anxiety were positively associated with increased follow-up levels of AUCg (β = 0.30, p = 0.001), greater pre-surgery perceived burden score was associated with steeper cortisol slope (β = 0.27, p = 0.017) after controlling for a wide range of covariates. Conclusion: These outcomes support the utility of psychological interventions aimed to increase the awareness of caregiving tasks and demands in informal caregivers.