Inclusion body myositis (IBM) is an idiopathic inflammatory muscle disease that predominantly affects elderly men over the age of 50 years, and the number of patients is rapidly increasing in Japan. Generally, muscle weakness and atrophy occur asymmetrically in the flexor muscles of the fingers and wrists and the quadriceps muscles. Invasive muscle biopsy is essential for the diagnosis of IBM. Although its pathogenesis is not yet understood, both inflammatory as well as degenerative mechanisms are postulated to be involved. In particular, degeneration of the IBM muscle may be associated with the IFN-II secretion by highly differentiated CD8+ T lymphocytes. Cytoplasmic 5'-nucleotidase 1A (cN1A) antibody has been detected in the blood samples of approximately half of the patients with IBM. While there are positive opinions about the diagnostic significance of the antibody, its usefulness for the diagnosis of IBM is limited. The results of passive immunization support its etiologic significance; however, more detailed verification, including active immunization, is needed in the future.