Fifteen Marine recruits with acute heat stroke were examined for (1) predisposing factors, (2) blood coagulation disturbances, (3) renal function abnormalities, and (4) blood composition alterations. Epidemiologic data identified the following risk factors; previous residence in a temperate climate, first phase of training, fatigue, and strenuous exercise in hot, humid conditions. Results of blood coagulation studies disclosed an increase in prothrombin and partial thromboplastin times, with a decrease in platelet count, probably indicating a transient, low-grade consumptive process. Blood urea nitrogen and creatinine levels and creatinine clearance were normal. Only mild elevations of SGOT, SGPT, and lactic dehydrogenase levels were noted, and in combination with clinical observations, they argued against significant muscle damage. No deaths or instances of renal failure occurred.