Muscle biopsy for diagnosis of malignant hyperthermia susceptibility in two patients with severe exercise-induced myolysis. Muscle biopsy and in vitro contracture tests for diagnosis of susceptibility to malignant hyperthermia (MH) were performed in two patients who had developed fever and severe myolysis during exercise. MH susceptibility was confirmed in one patient, but in the other, exercise-induced heat stroke proved to be the correct diagnosis. Clinical presentation and epidemiology of exercise-induced MH and its relation to the heat stroke syndrome are discussed. 