Proteolytic enzyme expression was studied by matrix metalloproteinases (MMP) immunoreactivity (-IR) in muscle biopsies from patients with amyotrophic lateral sclerosis (ALS), spinal muscle atrophy (SMA) and chronic axonal neuropathies (CANP). In normal muscle MMP-2-, MMP-7-, and MMP-9-IR was localized at neuromuscular junctions, in vessels and nerve branches. ALS biopsies of clinically non-affected muscles revealed neither MMP-2, -7-IR nor MMP-9-IR in atrophied myofibers. ALS-affected biopsies revealed MMP-9-IR, and to lesser extent MMP-2- and MMP-7-IR at normal sized and atrophied myofibers. Biopsies of SMA showed MMP-9- and MMP-7-IR at normal sized and atrophic myofibers, while MMP-2-IR was undetectable. In CANP MMP-9-IR was found at normal sized and atrophied myofibers. Distinct expression patterns of MMPs may thus reflect different stages of muscle denervation atrophy.