In order to evaluate the long-term-safety of halopredone acetate (THS-201: 17 alpha, 21-diacetoxy-2-bromo-6 beta, 9 alpha-difluoro-11 beta-hydroxy-1, 4-pregnadiene-3, 20-dione), a 52-week chronic toxicity study was performed on the basis of its local accumulation in dogs. In doses of 0.1, 0.5 and 2.5 mg/kg, THS-201 was injected into the right knee joint in both sexes of dogs every 2 weeks for 39 weeks and withdrawn for 13 weeks. In this study, the below slight local changes were observed in both sexes of dogs treated with 2.5 mg/kg/2 weeks of THS-201: focal loss of hair of the injection site, lesser stain in cartilage matrix of articular cartilage and meniscus in light microscopic examinations, and irregular thickness and elongation of collagen fibers, roughness of fibrous density and decrement of proteoglycans in electron microscopic examinations. In conclusion, systemic adverse effects were not observed in any dogs treated with THS-201.