Introduction: Cardiac arrest is often fatal and can be extremely stressful to patients, even if spontaneous rhythm is returned. The purpose of this study was to analyze the hormonal response after return of spontaneous circulation (ROSC). Methods: This is a retrospective review of the chart and laboratory findings in a single medical facility. The patients admitted to the intensive care unit after successful resuscitation after out-of-hospital cardiac arrest were retrospectively identified and evaluated. Patients with hormonal diseases, patients who received cortisol treatment, those experiencing trauma, and pregnant women were excluded. Serum cortisol, adrenocorticotropic hormone (ACTH), and anti-diuretic hormone (ADH (vasopressin)) were analyzed and a corticotropin-stimulation test was performed. Mortality at one week and one month after admission, and neurologic outcome (cerebral performance category (CPC)) one month after admission were evaluated. Results: A total of 117 patients, including 84 males (71.8%), were evaluated in this study. One week and one month after admission, 87 (74.4%) and 65 patients (55.6%) survived, respectively. Relative adrenal insufficiency, and higher plasma ACTH and ADH levels were associated with shock-related mortality (P = 0.046, 0.005, and 0.037, respectively), and ACTH and ADH levels were also associated with late mortality (P = 0.002 and 0.004, respectively). Patients with relative adrenal insufficiency, ACTH ≧5 pg/mL, and ADH ≧30 pg/mL, had a two-fold increased risk of a poor outcome (shock-related mortality): (odds ratio (OR), 2.601 and 95% confidence interval (CI), 1.015 to 6.664; OR, 2.759 and 95% CI, 1.060 to 7.185; OR, 2.576 and 95% CI, 1.051 to 6.313, respectively). Thirty-five patients (29.9%) had a good CPC (1 to 2), and 82 patients (70.1%) had a bad CPC (3 to 5). Age ≧50 years and an ADH ≧30 pg/mL were associated with a bad CPC (OR, 4.564 and 95% CI, 1.794 to 11.612; OR, 6.568 and 95% CI, 1.918 to 22.483, respectively). Conclusions: The patients with relative adrenal insufficiency and higher blood levels of ACTH and ADH upon ROSC after cardiac arrest had a poor outcome. The effectiveness of administration of cortisol and ADH to patients upon ROSC after cardiac arrest is uncertain and additional studies are needed.