Juvenile chronic segmental spinal muscular atrophy of Hirayama is described in two adult identical twins from the Department of Neurology, University of Vermont College of Medicine, Burlington, VT. In both patients examined at 69 years of age, the weakness was first noticed in the right hand at age 16 and within six months there was similar weakness of the left hand and atrophy of muscles in both hands. The disability progressed over the next four years but after age 21, further decline was barely noticeable. Examination at age 69 showed marked atrophy and weakness of the intrinsic muscles of both hands, the ulnar half of the forearm flexors, and of the brachioradialis muscles more on the right side. Occasional fasciculations were noted in involved muscles. The triceps and Achilles reflexes were decreased in one patient and the tendon reflexes were otherwise normal. Nerve conduction studies showed reduced amplitude of the ulnar and median compound muscle action potential, mildly slow conduction in upper and lower extremity motor nerves, and mild prolongation of the F-wave latencies. EMG showed no fibrillations, positive waves or fasciculations. The motor units in the upper extremity muscles showed neurogenic features. Based on the identical sex, phenotypes, blood groups, and HLA typing in the two brothers there was a 98.8% calculated chance that these twins were identical. [1]