Systemic lupus erythematosus with polymyositis and polyarthritis was diagnosed in a 7-year-old female Standard Poodle. Pertinent clinical signs included extreme muscular weakness, muscle wasting, atrial fibrillation, and dysphagia due to megaesophagus. Aspiration pneumonia secondary to the megaesophagus contributed to the death of the dog. Serum muscle enzyme activities were increased. Electromyographic findings included fibrillation potentials, positive sharp waves, increased insertional activity, and bizarre high-frequency repetitive potentials. Histopathologic findings in skeletal muscle included myofiber necrosis and phagocytosis; regeneration of myofibers; perivascular and interstitial infiltrations of macrophages, lymphocytes, and plasma cells; and type I and II myofiber degeneration and vacuolation.