In three adult men, serum creatine kinase activity was constantly raised for at least 4 years. They had been normal in other neuromuscular functions and did not have any established disease. Quantitative morphologic and pharmacologic studies were performed on biopsied muscle. The biceps brachii of patient 1 contained 0.3% necrotic fibers. In patient 2, only slight variation of muscle fiber diameter was noted. Muscle of patient 3 contained a few small angular fibers, and 11% of fibers exhibited internal nuclei. Sensitivity to caffeine in vitro was increased in patients 2 and 3, as seen in survivors of malignant hyperthermia; patients in hyperCKemia may be susceptible to malignant hyperthermia.