A 23 year-old man was admitted because of weakness and atrophy in both tights. He was 184 cm in height with arachnodactyly and had a mild dilatation of the ascending aorta which was detected by cardiac echogram and chest CT scan. A laboratory examination revealed normal serum values for mucopolysaccarides and amino acids, and 46XY chromosomes. These findings indicate a Marfan's syndrome phenotype. A neurological finding showed a proximal dominant muscle atrophy and increased deep tendon reflexes. The EMG revealed a polyphasic action potential, and a subsequent muscle biopsy demonstrated a small group atrophy and atrophy of Type II fibers. Neuroradiological studies of brain and spinal cord indicated no substantial abnormalities. Only 4 cases of Marfanoid habit accompanied by primary nervous system disturbances have been reported and these had variable complications or severe neurological deficits. The present case, which involved mild neurogenic muscle atrophy and brisk deep tendon reflexes without complications except myopia, was unique.