摘要The clinical behavior of nonossifying fibroma (NOF) appears to be a benign lesion without recurrence or canceration.[1] Ewing's sarcoma (ES) is usually regarded as a differential diagnosis of NOF.[2] There has not been any report on NOF and ES appearing in the same position simultaneously. Here, we first report a case that the simultaneous onset of two entities was mistaken for the development of NOF into ES. A17?year?old boy referred to hospital in October 2010 for a durative ache in his right knee after an injury during football training. Initial radiographs revealed a low?density area with sclerotic margin on the proximal part of the right tibia. Computed tomography (CT) scan showed a radiolucent, eccentric, and cortically based lesion in the posterolateral metaphysis of the right tibia. Then, the patient underwent magnetic resonance imaging (MRI), in which a lesion revealed long mixed signs on T1?weighted imaging (T1WI) and short signs on T2WI accompanied with bone contusion around it. These reports suggested that the eccentric area could be an osteoid osteoma or a NOF. A percutaneous biopsy of the right proximal tibia lesion was performed, and then NOF was pathologically confirmed. Hence, curettage and bone grafting were performed.