After a historical review of the problem of progressive myoclonic epilepsy, the author makes a detailed clinical and genetic description of 5 families from the Valais canton, affected with the disease. The metabolic disturbances observed in these cases (urinary elimination of mucopolysaccharides) are given particular attention. The author further discusses the various classifications of myoclonic epilepsy which were established on the basis of clinical symptomatology, electroencephalographic tracings, pathological anatomy and biochemical findings.