A 37-year-old man was transferred to our emergency center because of heat stroke with circulatory shock. Despite aggressive body cooling, massive intravenous transfusion, and supply of inotropic agents, shock was persistent. To evaluate adrenal function, an adrenocorticotropic hormone stimulation test was conducted and the results indicated that he had critical illness-related corticosteroid insufficiency (CIRCI) as a result of adrenal insufficiency. Continuous hydorocortisone administration was started and he recovered from shock within a few hours. He was discharged on the thirty-seventh hospital day. Serum cortisol and cytokine concentrations were initially high and the cytokines decreased subsequent to hydrocortisone administration. It is speculated that CIRCI is an exacerbating factor in heat stroke, and hydrocortisone may be a potential therapeutic approach in such patients.