The present paper describes the clinical, electrophysiological and histological findings made in 12 patients belonging to 2 genealogical lineages in Switzerland, suffering from paramyotonia congenita. This is the first report of this disease in Switzerland. The myopathy, transmitted by autosomal dominant inheritance, is characterized by a typical past medical history and by the persistent contraction of the muscles of the face, arms and legs provoked by exposure to cold. Also of diagnostic importance are the myotonic reactions inducible at room temperature, such as percussion myotonia, active myotonia and paradoxical myotonia. During paramyotonic episodes provoked by exposure of the subjects in a refrigeration chamber, serum potassium concentrations remained within normal limits. Potassium loading producing serum levels above 6 mEq/1 in 2 patients gave rise to stiffness and weakness of the arms and legs, but no signs of paralysis. Induced hypokalemia in 3 cases caused no paramyotonic symptoms. Electromyographic recordings in 5 patients showed myotonic discharges, which disappeared upon cooling of the limb, giving way to progressive muscular stiffness. Histological, histochemical and electronmicroscopical examination of the muscle tissue revealed only diagnostically unspecific myopathological changes.