Inclusion body myositis, toxic myopathy, necrotizing myopathy and myotonic dystrophy should be excluded in the diagnosis of polymyositis, because the Bohan diagnostic criteria do not distinguish polymyositis from these diseases. 2, polymyositis is a relatively rare disease, and inclusion body myositis is more common than it is. 3, Endomysial inflammation is not unique to polymyositis, but is also seen in non-immune myopathies such as myotonic dystrophy, toxic, and metabolic myopathies. 4, Muscle biopsy should not be performed within 1 month after needle electromyography has been performed. 5. For patients with clinical manifestations of fatigue and increased serum transaminases or lactate dehydrogenase, CK must be performed to exclude muscle-derived transaminase elevation to avoid misdiagnosis as liver lesions and liver biopsy. 6. Symptoms of muscle weakness must be present during the active phase of polymyositis, and clinical manifestations of myalgia with normal muscle strength cannot diagnose polymyositis. 7. The diagnosis of polymyositis is suspicious when a muscle biopsy cannot show primary inflammation (CD8+/MHC-1 complex). 8. The goal of treatment for inflammatory myopathies is to improve muscle strength. CK can be used as an indicator of myositis activity, but is not a therapeutic goal. 9, If treatment for polymyositis only reduces CK but does not improve muscle strength, the diagnosis should be re-evaluated, and re-staining of muscle biopsy specimens for reading, and secondary biopsies may be considered. 10. When the patient’s muscle strength improves but does not fully recover, treatment with immunosuppressants or alternate-day prednisone should be continued.