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Background: Nodular fasciitis most often occurs within subcutaneous tissues, but may also arise within skeletal muscle, dermis, vessels, peripheral nerves and, although rarely, within joints. Knowledge regarding the cause of aneurysmal bone cysts, its natural history, and the results of treatment is limited. Secondary aneurysmal bone cysts are associated with other neoplastic processes. Intra-articular nodular fasciitis in the elbow joint has not been reported previously, nor has the development of aneurysmal bone cyst secondary to intra-articular nodular fasciitis in the elbow joint., Case Presentation: We report an unusual case of a Japanese 13-year-old boy who presented with a 1-year history of right elbow pain. The onset of pain was insidious, without antecedent trauma. On physical examination, the range of motion of the elbow was limited. Grip strength was reduced in the affected extremity. Incisional biopsy was performed and histologic findings revealed nodular fasciitis in the elbow joint. After tumor excision, a secondary aneurysmal bone cyst in the distal humerus developed. Endoscopy-assisted curettage and artificial bone grafting were performed. One year after surgery, a plain radiography showed no recurrence, and the patient returned to his daily activities without any symptoms., Conclusion: An aneurysmal bone cyst in the distal humerus developed after excision of intra-articular nodular fasciitis arising in the elbow. The secondary aneurysmal bone cyst successfully healed after endoscopy-assisted curettage and artificial bone grafting. The findings of this case suggest that these two tumors reside in the same biologic spectrum defined as USP6-induced tumors.