Benign, monomelic juvenile amyotrophy of the hand (Hirayama type): new data.--Within the space of a few months a 21-year-old man developed isolated amyotrophy and paresis of the right hand; thirteen years later the clinical picture was unchanged. Medullo-cervical MRI was normal. The EMG confirmed the involvement of the anterior horn. Unilateral chronic distal juvenile amyotrophy located in an upper limb (Hirayama type) was diagnosed on clinical findings. But the spread of electrical abnormalities to the lower limbs suggested a transitional form between this syndrome and other disorders of the anterior horn.