Unicameral bone cysts have been treated by variable surgical methods, but reported with high recurrent rates. Recently, the treatment of unicameral bone cyst by topical injection of methylprednisolone acetate has been reported with excellent results. The authors treated 4 cases of the unicameral bone cyst by topical injection of methylprednisolone acetate and followed up for 3 years and 5 months to 4 years. The results were as follows: l. 3 cases were healed completely with obliteration of cyst cavity and 1 case was in residual healing process. 2. If the cavity persists or recurrs, it seems possible to cure the cyst by repeated local injection.