A previously healthy 18-year-old male, following appendectomy developed post-anaesthetic hyperthermia (42.1 degrees C) with an elevation of serum creatine kinase and activated partial thromboplastin time. Repeated arterial blood gases were normal. Cooling and anti-pyretic medication did not control the fever. In contrast, sodium dantrolene appeared effective in lowering the patient's temperature and normalizing the vital signs, both acutely and over the following three days. Subsequent muscle biopsy revealed a normal contracture response to caffeine alone or in the presence of halothane. However, the muscle had a larger than normal potentiation of evoked twitch tension in the presence of caffeine and halothane. Electrophoresis of the muscle revealed a marked increase of an unidentified low molecular weight protein. The patient's clinical course, and the results of the muscle studies, suggest that an abnormality of skeletal muscle.