Polymyositis is a systemic, noninfectious, possibly immune-mediated, inflammatory muscle disorder in adult dogs. It may be acute or chronic and progressive. Clinical signs include depression, lethargy, weakness, weight loss, lameness, myalgia, and muscle atrophy. CK may be increased, and electromyography reveals abnormal spontaneous muscle activity. Muscle biopsy reveals myonecrosis, lymphocytic-plasmacytic perimuscular infiltration, phagocytosis, and fiber regeneration. Polymyositis may be associated with megaesophagus and immune-mediated disorders (myasthenia gravis, lupus erythematosus, polyarthritis). Oral corticosteroids (1–2 mg/kg, twice a day for 3–4 weeks) are the treatment of choice; other immunosuppressive agents such as azathioprine or cyclophosphamide can also be used. Prognosis is favorable, although relapses are not uncommon.