[F]fluorodeoxyglucose positron emission tomography/computed tomography ([F]FDG-PET/CT) is useful for diagnosing cancers and inflammatory diseases. A polymyositis/dermatomyositis (PM/DM) lesion is an inflammatory heterogeneous disease of the striated muscle. In the present study, the maximum standardized uptake value (SUV) was compared between 22 cases with definite or probable PM/DM (PM/DM group) that underwent [F]FDG-PET/CT examination and the same number of patients with no myopathy. The average proximal muscle FDG uptake value (SUV) for each patient was represented by calculating the average of the SUV for these muscles bilaterally. The correlation between creatine kinase (CK), serum creatine kinase isoenzyme, myodynamia of the proximal limb girdle muscle and SUV of each muscle group were analyzed. The results indicated that the SUV was markedly different between the PM/DM group and the non-myopathy group. It was demonstrated that [F]FDG-PET/CT has a diagnostic value for PM/DM. The serum CK levels and the SUV were negatively correlated with myodynamia. [F]FDG-PET/CT may be used for examination to assess the severity of myositis. Furthermore, it may provide detection sites for muscle biopsy in patients with myositis.