A new type of progressive muscular dystrophy, autosomal recessive distal muscular dystrophy, is described, based on observations on 17 cases (8 males and 9 females) in 8 families, including an autopsied case. The disease developed in young adults. Muscle weakness and atrophy were most marked in the distal parts of the legs, especially in the gastrocnemius and soleus muscles, and then spread to the thighs and gluteal muscles. Early impairment of standing on tip-toe with retention of the ability to stand on the heels was conspicuous. Difficulty in climbing stairs, standing up and walking subsequently appeared, but rarely progressed to confinement to bed. The forearms became mildly atrophic, with decrease in grip strength, but the small hand muscles were spared. The EMG showed myopathic changes and nerve conduction was normal. Serum creatine kinase activity was characteristically increased up to 100-fold in the early stages of the disease. It was also markedly increased in subjects in the preclinical stage and mildly in some heterozygotes. Muscle biopsies revealed myopathic changes with severe segmental necrosis accompanied by regeneration. The changes were similar to those of Duchenne muscular dystrophy. An autopsied case, aged 68 years, showed generalized muscle abnormalities with a distal predominance. The muscles in the lower legs, especially those of the calves, were severely affected. No lesions were found in the brain, spinal cord or peripheral nerves.