Polymyositis (PM) has been a common clinical diagnosis for inflammatory myopathies without dermatitis, since Bohan and Peter established diagnostic criteria classifying polymyositis and dermatomyositis by the presence of dermatitis. In Japan, the diagnostic criteria of polymyositis and dermatomyositis for medical subsidies are also based on the presence of dermatitis, and there are a number of patients with “polymyositis”. However, clinical, pathological, and serological studies have revealed that the inflammatory myopathies without dermatitis are a heterogenous group, including sporadic inclusion body myositis (sIBM), immune–mediated necrotizing myopathy (IMNM), collagen disease related myositis, and PM. For adequate clinical decisions as well as high–quality studies, PM should be diagnosed based on strict diagnostic criteria including pathological examination, such as the European Neuromuscular Centre (ENMC) criteria. Among 972 cases that had been diagnosed as inflammatory myopathies in our laboratory between 2000 and 2019, about 2/3 cases (550/925) lacked dermatitis, and could be diagnosed as “polymyositis” by Bohan and Peter criteria. However, based on the ENMC criteria, only 1.7% (17/972) cases were classified as PM. Even after the diagnosis of PM was made by pathological examination, some patients may be reclassified as other diseases, especially as sIBM. The improvement of pathological examination further reduced the number of misdiagnosed “PM”. In addition, infiltration of CD8 positive cells surrounding non–necrotic fibers, a hallmark of PM and sIBM, is also observed in other myopathies. Some researchers doubt the existence of PM. Thus, PM is a shrinking entity.