Extremely favorable results are obtained with the use of microcrystals of methylprednisolone acetate for treatment of bone cysts. These results have led to a complete suspension of surgical treatment of bone cysts since 1974. On the basis of these results with corticosteroids, a surgical procedure that involves an incision at the fracture location and bone graft inserted is not indicated. Results following surgery indicate a recurrence rate of approximately 25% to 30%. Since we are as yet unable to explain the mechanism by which the local injection of MPA promotes bone replacement of the cyst, the present observations only reaffirm the hypothesis (presented in our early publications) that the corticosteroid exerts a destructive action on the pathological tissue of the lesion, thus favoring a progressive process of repair. We consider this explanation valid even for lesions, e.g., eosinophilic granulomas and nonossifying fibromas, in which this method of treatment has had varying degrees of success.