Electron microscopic studies of long-term denervated rat muscles have identified very small, immature myofibers that are believed to arise from detached satellite cells that have fused to form new fibers within the interstitial space. At present, it is unknown whether and to what extent equivalent fibers exist in denervated human muscle. Serial sections of muscle biopsies from 66 patients diagnosed with polyneuropathy or amyotrophic lateral sclerosis were immunolabeled with anti-NCAM and anti-neonatal myosin heavy chain monoclonal antibodies that are both neurally and developmentally regulated. We evaluated 200 myofibers in each section. Of the biopsy specimens, 75% contained small myofibers that showed a thin perinuclear cytoplasmic rim. Small fibers expressing neonatal myosin heavy chain (MHCn+) were found in all of these biopsies (100%) and NCAM+ fibers in 98%. The percentage of MHCn+ small fibers averaged 82% and NCAM+ small myofibers averaged 40%. The percentage of NCAM+ small fibers was significantly lower than that of MHCn+ fibers. In contrast, the percentage of MHCn+ vs. NCAM+ angular atrophic fibers did not show a significant difference. A substantial subset of neurogenic biopsies showed small fibers that differ from angular atrophic fibers both in size and expression pattern of MHCn and NCAM. Myogenesis appears to be a frequent finding in neurogenic atrophy.