For years I have been studying a group of patients who have presented a well characterized, uniform and unchanging clinical picture. The only objective manifestation has been atrophy of the outer, radial portion of the muscles of the thenar eminence. My attention was first drawn to the problem by a patient aged 26 who presented this syndrome and whose condition was diagnosed by a neurologist as "progressive muscular atrophy." Examination of this patient and the later course of his disease proved that the disorder was not progressive muscular atrophy. My group of patients had presented diagnostic difficulties, as they were referred to the clinic with the following diagnoses: multiple sclerosis, syringomyelia, gliosis spinalis, occupational neuritis of the median nerve and neuritis of the brachial plexus. Seven patients presented this syndrome in a clearcut form and were examined completely in the clinic or in the hospital. In addition, about 10 patients