Sporadic inclusion body myositis (sIBM) is the most common form of myopathy with inflammation in those over the age of 50 years in Western countries [1, 3, 5, 7]. The prevalence in Caucasians is 4.9–14.9 per million, but 1.07 in Turkey [6]. The prevalence of sIBM in Asian people including Japanese has not been examined. Several mechanisms of sIBM are proposed, for example, betaamyloid accumulation, immune system abnormalities, viral infection, genetic background [1, 8]. However, none of these are concluded to be the specific cause of sIBM. We have now performed a retrospective survey of Japanese patients of sIBM diagnosed at the National Center of Neurology and Psychiatry (NCNP). The increasing numbers of sIBM patients may suggest the clue to elucidate the pathomechanism of sIBM. Only patients with ‘definite’ or ‘probable’ sIBM by the clinical and biopsy criteria [7] were included in the analysis. Biopsies were re-evaluated, and were confirmed the pathological diagnosis of sIBM. We also used revised Bohan and Peter criteria for diagnosis of polymyositis (PM) [5]. In NCNP, the first patient of sIBM was diagnosed in 1989, and the number of patients diagnosed has been increasing year by year, especially after 2002 (Fig. 1). A total of 77 sIBM patients were identified between 1990 and 2007. The average age of onset in sIBM in Japan was 63.4 years old. The numbers of patients with sIBM and PM between 1999 and 2007 were 69 and 165, respectively (Table 1). Accordingly, the number of sIBM patients is estimated to be half that of PM. Given the number of PM patients in the national survey in 2003 (approximately 3,000 patients) in Japan, the number of sIBM is estimated to be around 1,250. Therefore, we assess that the prevalence of sIBM in Japan is 9.83 per million in 2003. The numbers of sIBM and PM between 1990 and 1998 were 8 and 151 patients, respectively. As the number of PM patients in the national survey of 1991 was still around 3,000, the prevalence calculated by the same method was 1.28 per million in 1991, suggesting an increase in the number of sIBM in Japan. We also examined the relationship between birth year and the number of sIBM patients diagnosed in NCNP since 1978 (Fig. 1b). The numbers of sIBM patients are increasing in a linear manner among the individuals born after 1920s. The etiology of sIBM is not yet known and still under discussion in either a primary inflammatory myopathy or a primary degenerative myopathy with a secondary inflammatory disease. The lack of significant clinical response with various immunosuppressants is against sIBM being a primary autoimmune disorder. Accumulation of betaamyloid in rimmed vacuoles is interpreted as a primary N. Suzuki M. Aoki (&) Department of Neurology, Tohoku University School of Medicine, 1-1Seiryo-machi, Aoba-ku, Sendai 980-8574, Japan e-mail: aokim@med.tohoku.ac.jp