Sporadic inclusion body myositis (sIBM) is a chronically progressing inflammatory myopathy most common in the aged population. Asymmetric muscle weakness and waste of the quadriceps and finger and wrist flexor muscles are characteristic features of sIBM. Histological findings suggest the involvement of a crosstalk of inflammatory and myodegenerative mechanisms in the pathogenesis of sIBM. As an etiological clue to sIBM, identification of autoantibodies against cytosolic 5'-nucleotidase 1A (NT5C1A) in plasma and serum samples from patients with sIBM has been attracting attention. So far, various methods with clinical utility have been established to detect anti-NT5C1A autoantibodies. The measurement of the autoantibodies is useful for the diagnosis of sIBM due to its high specificity. Moreover, the autoantibodies may have pathogenic roles in the development of the disease by stimulating catabolic conditions and/or causing dysfunction in protein degradation of skeletal muscles; however, the molecular mechanisms by which the sarcoplasmic autoantigen is recognized and involved in the degeneration of myofibers remain unclear.