The myotonias are a group of uncommon disorders. All display a characteristic electromyographic picture and some of the disorders have systemic manifestations. They are important to the anaesthetist for several reasons. Anaesthetic and surgical interventions may induce myotonia which, when initiated, may complicate the course of anaesthesia and be difficult to abolish. Furthermore, patients often display extreme sensitivity to some drugs and abnormal reactions to others. During anaesthesia, the extramuscular manifestations that occur in some myotonic disorders may assume greater importance than the myotonia, but may be masked before operation by the patient's reduced exercise tolerance. Finally, patients with myotonic disorders may be susceptible to malignant hyperpyrexia and have an increased incidence of malignant hyperpyrexia-like reactions.