PURPOSE: To evaluate radiographic changes and clinical effects after the injection of steroid injection into simple bone cyst. MATERIALS AND METHODS: We analyzed plain radiographic and CT findings after the injection of steroid (methylprednisolone acetate, MPA) into simple bone cyst. Twelve patients were involved; eight were males and four were females, and their ages ranged from 6 to 41 years. They were treated from one to seven times with an intracystic injection of MPA, 80-200mgs per injection. All patients were evaluated by plain film, and four by CT, and the mean follow-up period was 20 months. RESULTS: Postinjection plain radiographic findings were as follows: increased internal radiodensity (n=10), smaller cyst (n=8), cortical thickening (n=7), and radiodensity of double ring pattern (n=4). CT findings were as follows: increased internal attenuation (n=4), smaller cyst (n=4) and radiodensity of double ring pattern along the cyst wall (n=4). All patients improved clinically and radiologically, especially those with radiodensity of double ring pattern. CONCLUSION: The injection of steroid into simple bone cyst is an effective treatment. A new radiographic finding after injection is radiodensity of double ring pattern along the cyst wall, and we believe that this indicates progression of the healing process.