Nogo (RTN4) belongs to the reticulon (RTN) family of integral membrane proteins. RTN4A (Nogo-A), RTN4B (Nogo-B) and RTN4C (Nogo-C) are isoforms of RTN4. In the gastrocnemius muscle of transgenic mice bearing an SOD1 mutation ("ALS model"), increased Nogo-A mRNA and protein was reported, and similar changes were reported in muscle biopsies of patients with amyotrophic lateral sclerosis (ALS) but not with peripheral neuropathy or primary muscle diseases, leading to the proposal that Nogo-A in skeletal muscle is a new specific molecular marker of ALS. Here we report, based on studies of muscle biopsies from patients with ALS, peripheral neuropathies, polymyositis, dermatomyositis and morphologically nonspecific myopathies that, in addition of strong Nogo-A immunoreactivity within apparently-denervated small angular fibers in ALS and peripheral neuropathies, Nogo-A was strongly immunoreactive within desmin-positive regenerating muscle fibers in various myopathies, and its expression on immunoblots was increased in all those neuromuscular diseases. In conclusion, we have found that the presence of Nogo-A in diseased human muscle biopsies is not limited to ALS.