The author reports a clinical case of Isaacs-Mertens syndrome (syndrome of continuous muscle fibre activity, neuromyotonia, pseudomyotonia). A 55 years old man had increasing stiffness, painful spasms and myokymias of the peripheral muscle, associated with hyperhidrosis and absent tendon reflexes. Electromyography showed continuous electrical activity while at rest, general anaesthesia, after intravenous injection of diazepam and after peripheral nerve block. The activity disappeared after muscle infiltration of procaine and after intravenous injection of succinylcholine. After two years treatment with carbamazepine, 600 mg daily, the patient showed a normal clinical and neurophysiological picture. The etiology of the disease is unknown. The defect probably lies in the terminal motor network. A review of previous literature shows a typical uniformity of the electromyographic findings and of improvement on phenytoin and carbamazepine therapy, while there is a relative polymorphism of the clinical features.