It is reported a case of a 61 years-old man with progressive asthenia, disfagia, disphonia and diplopia, of variable intensity during the day, who had a very good response to anticolinesterasic drugs and corticoesteroids. The repetitive estimulation tests at low frequency, resulted in large increment (maximum 275%) of the basal voltage after exercise. At high frequency he also had a large increment. Radiological and laboratory investigation three times in a seven-year period was normal, without evidence of any neoplasia. Muscle biopsy showed a type II muscle fiber atrophy. The repetitive estimulation tests repeated three times, was typical of myasthenia gravis in one ocasion and in the other two, typical of myasthenic syndrome (increment of 418%). A discussion about other cases with similar findings is made after a review of the literature.