This study quantitatively assessed skeletal muscle histopathology in 57 patients with inflammatory myopathy, including 20 patients with polymyositis (PM), 19 patients with dermatomyositis (DM), and 18 patients with evidence of an additional connective-tissue disease. No histologic criteria for invariably distinguishing patients with inflammatory myopathy were established because of overlap in individual measurements, but general histopathologic distinctions were confirmed. In PM, endomysial mononuclear cell infiltration (fibers bordering on inflammation) was usual, whereas in DM inflammation of large vessels, fibers with circumscribed areas of myofibrillar loss, and perifascicular atrophy were seen. Patients with evidence of an additional connective-tissue disease were most similar to the DM patients, with a greater prevalence of perivascular inflammation than in the PM patients. Because of varying histopathology (and presumed varying pathogenesis), future therapeutic trials would be more informative if they were designed using patients with homogeneous histologic features.