We report a case of a healthy, young, female soccer player who developed progressive pain in her right hip. A bone cystic lesion was found in the right femoral neck and proximal femur. The lesion was considered a benign bone cyst and the patient was treated with injections of autologous bone marrow and grafting into the femoral neck. However, the cystic lesion did not heal. Subsequently, the patient fell and developed a fracture of the femoral neck that required internal fixation. The bone biopsy showed characteristic histologic features of fibrous dysplasia. A bone scan showed other areas of suspected dysplasia. Pamidronate therapy was given, and a reduction of the increased uptake was seen on bone scans. Fibrous dysplasia must be considered in the differential diagnosis of any cystic bone lesion.