ABSTRACT— The phosphomannosyl receptor system is responsible for both the receptor-mediated endocytosis and the intracellular transport of lysosomal enzymes. In the present study this receptor system was examined in affected muscles of patients with various neuromuscular diseses. The total activity of ß-N-acetyl-glucosaminidase, a marker enzyme of lysosomal hydrolases, was significantly elevated in the patients with myopathies (polymyositis and muscular dystrophies) but only slightly increased in those with neurogenic muscle atrophies (amyotrophic lateral sclerosis, polyneuropathy or other neurogenic muscle disease). The increase was most prominent in the group of polymyositis. The content of phosphomannosyl receptors was increased in the patients with myogenic muscle disease but not in those with neurogenic disease. The receptor binding of lysosomal enzymes was saturable and inhibited with mannose 6-phosphate showing the typical characteristics of phosphomannosyl receptors. The characteristics of the receptors were very similar both to control and to diseased muscle samples. When surveying all the material, the content of phosphomannosyl receptors correlated highly significantly with the muscular activity of ß-N-acetylglucosaminidase, muscle atrophy index, and serum creatine kinase activity.