A myasthenic syndrome was diagnosed in 3 female patients aged 37, 47 and 56 years. The symptoms were first noticed in childhood, and at 16 and 25 years. Respectively weakness and atrophy of finger extensor muscles were present in all patients. In 2 of them scapular and cervical muscles were involved. Weakness was markedly increased by cold in every patient. Similar symptoms were noted in first degree relatives in all cases. Single nerve stimulus elicited a repetitive compound muscle action potential in hand muscles. Repetitive nerve stimulation induced a myasthenic decrement in finger extensor muscles. SFEMG studies demonstrated increased jitter with frequent blockings. Genetic, clinical and electrodiagnostic data were consistent with the hypothesis of the so-called "Slow-Channel" myasthenic syndrome. As 2 of these patients were considered to have "unknown myopathy" the use of careful nerve stimulation tests is advocated in such cases.