Objective: To assess adrenal function in patients undergoing coronary artery bypass grafting (CABG) by means of the low-dose (1 mg) ACTH test, and to correlate the adrenal function with clinical outcome. Methods: During a 5-month period we prospectively included 45 patients undergoing elective CABG with cardiopulmonary bypass and without symptoms of endocrine disease. Low-dose (1 mg) ACTH tests were performed on the day before surgery (day 21), immediately after the operation (day 0), on the two subsequent days in the intensive care unit (day 1 and day 2), and on the day of discharge from the hospital. A number of clinical, hemodynamic and laboratory parameters were monitored throughout. Results: On day 2 1, 75% of the study patients had normal stimulated plasma cortisol concentrations. Eleven patients (25%) had an impaired adrenal response to 1 mg ACTH. The stimulated plasma cortisol concentrations in patients who had an inadequate adrenal response on day 2 1 remained significantly reduced on day 1 (756^205 vs 949^259 nmol/l, P ¼ 0.03) (mean^S.D.), day 2 (644 (580 -793) vs 885 (713-1087), P ¼ 0.03) (median (interquartile range)), and on the day of discharge (698^201 vs 854^186, P ¼ 0.05). In patients with a normal adrenal response in the preoperative setting peak cortisol concentrations were reached on day 1, in patients with a blunted adrenal response they were reached on day 2. There were significant correlations between the stimulated plasma cortisol concentrations and the blood loss (r ¼ 2 0.50, P ¼ 0.002) and volume balance (r ¼ 0.41, P ¼ 0.015). Conclusions: Occult (partial) adrenal insufficiency is common in patients undergoing CABG who are otherwise asymptomatic as regards endocrine disease. The adrenal function in these patients differs both in the magnitude of cortisol response to ACTH and in the time course, with significantly delayed peak cortisol concentrations. Adequate regulation of volume balance and the amount of blood loss seem to correlate with adequacy of adrenal function.