The objective of this study was to evaluate the role of MR examination in showing the effects of intralesional steroid injection on bone cysts. Nineteen patients with radiologically suspected (group A, n=15) or already treated (group B, n=4) bone cysts underwent MRI (0.5 T; protocol: T2* gradient recalled echo; short tau inversion recovery or inversion recovery fast spin echo; T1 SE before and after contrast administration, 0.2 mmol/kg). All patients underwent steroid therapy for bone cysts and were followed up for 6-18 months after treatment. Evaluation of bone cyst response to therapy was based mainly on the detection of reparative tissue outlining all cystic portions at first MRI control. In group A, as early as 1 month after the first injection, the presence of a thin reparative tissue lining the cystic wall was observed. Follow-up studies revealed the progressive thickening of this reparative tissue and new bone formation. Similar features were present in the cysts of group B. Residual cystic cavities were depicted in 7 cases, with no evidence of enhancing tissue, requiring further treatment. The formation of a reparative tissue is the first feature of bone cyst recovery after intracavital steroid injection and is appreciable only on MRI. Because of the optimal visualisation of this reparative tissue and of residual cystic cavities, MRI could be proposed as a non-invasive, alternative and effective tool for evaluating the efficacy of steroid injection therapy of bone cysts.