A global, randomized, double-blind placebo-controlled study was conducted to confirm that BYM338 (bimagrumab), an anti-activin type II receptor antibody, improves motor function in patients with sporadic inclusion body myositis after 52 weeks' treatment consisting of intravenous administration every 4 weeks at doses of 10, 3, and 1 mg/kg. In a Japanese sub-population (20 patients in total, 5 per dose group), no significant differences in the change from baseline of the 6-minute walking distance at Week 52 (primary endpoint) were observed between the placebo group and each BYM338 dose group. Furthermore, the lean body mass as an indicator of skeletal muscle mass increased in all BYM338 groups compared with the placebo group and the effects were dose-dependent. Overall, the Japanese sub-population showed similar trends as observed in the entire population (251 patients in total).