Malignant hyperthermia (MH) is a pharmacogenetic disorder. It is classically described as a hypermetabolic state triggered by halogenated anaesthetics and/or depolarizing muscle relaxants. In fact, since Denborough and Lovel's case, it has been shown that MH has a great number of clinical forms. The overwhelming picture of muscular hypercatabolism with fulminating hyperthermia and generalized rigidity is becoming rare. A better knowledge of the first symptoms explains in part the better prognosis : masseter spasm after suxamethonium, an increase in expired CO 2 concentration, unexplained tachycardia, ventricular arrhythmias. The use of dantrolene reduced the mortality of MH. The different types of clinical manifestations are due to genetic differences, the concentration of the anaesthetic agent, and the length of time of exposure to the drug. The severity of the episode is linked to environmental factors such as stress, physical exercise, ambient temperature, concomitant use of other drugs. Masseter spasm after suxamethonium is specific for MH, but not pathognomonic. It occurs in 1 % of cases in children when using halothane with suxamethonium. However, in those patients who displayed such a spasm, more than 50 % had a positive contracture test. Masseter spasm is often associated with severe rhabdomyolisis in patients with muscle dystrophy, especially Duchenne's dystrophy. In the latter case, major cardiac problems may occur at the time of anaesthetic induction. Even if there are no other signs of MH, all patients who have had a masseter spasm must be considered as open to doubt, and should be further explored. MH is often difficult to diagnose in medium severity types. When one symptom is dominant, such as a postoperative hyperpyrexia with minor signs of hypermetabolism in unsuspected patients, it can mislead the anaesthesiologist. MH can result in dubious or insidious manifestations. The number of cases diagnosed postoperatively is on the increase. Minor episodes of MH can be seen in susceptible patient (MHs) during or after an anaesthetic with non-trigger drugs. This is probably similar to MH crises seen during prolonged exercise in a hot and humid atmosphere. This is simular to effort malignant hyperthermia. It cannot be demonstrated that these syndromes, as well as the neuroleptic malignant syndrome, have anything in common. The only sure proof of belonging to the MH myopathy group is a positive in vitro contracture test. Epidemiological studies have defined a population potentially at risk. It includes those patients who survived a MH episode, parents of MHs patients, patients with evocative muscle and skeletal anomalies and or a predisposing myopathy. Not using triggering agents in these patients does not prevent one from closely monitoring them, especially body temperature and expired CO 2 concentration. Dantrolene should be administred as soon as there is some suspicion. Moreover, this drug has been shown to be efficient in other syndromes which look closely like MH, such as thyrotoxicosis. Any delay in starting treatment because of an unknown diagnosis may be fatal.