Purpose: To evaluate the effectiveness of steroid injection for treatment of simple bone cyst in adults. Materials and Methods: 11 patients who were able to be followed up from February 1998 to February 2003. These lesions were attained by needle aspiration or trepanation and 80 mg to 200 mg of Depo-medrol (methylprednisolone acetate, Shin Poong Pharm. Co., Korea) was injected. The follow-up roentgenograms were done every 2 weeks, and if there were no evidence of recovery, re-injection was done 8 weeks later. Injection has done maximally 5 times, and we evaluated the result with modified Neer, Pentimalli, Scaglietti and Sandra classification. Results: There were no cases in which one Injection was enough for full recovery, and 3 cases after 2 injections, 3 cases after 3 injections, 2 case after 5 injections had good result. but 2 cases after more than 5 injections did not show improvements and are currently under observation. One case showed a increase in size of the bone cyst after 2 injections of steroid and underwent curettage and autogenous bone graft. Overall outcomes by Pentimalli classification were good in One case, fair in 7 cases and poor in 3 cases at final follow-up. Conclusion: Local injection of steroids is effective method in adults and can be considered as a first line treatment modality.