A Japanese 13-year-old boy presented with a 1-year history of right elbow pain. The onset of pain was insidious, without antecedent trauma. He reported pain on both flexion and extension of the elbow. He had no medical history of disease, and his family had no history of musculoskeletal disease. On physical examination, flexion and extension of the elbow was limited to a range of 116°–30°. Grip strength was reduced in the affected extremity to 16 kg, compared to 21 kg on the left. Neurovascular examination showed normal results, with no lymphadenopathy or other enlargements. Plain radiography and computed tomography (CT) of the right elbow showed scalloping of the anterior portion of the distal humerus. Magnetic resonance imaging (MRI) showed multiple soft masses measuring 3 × 2 × 2 cm in the anterior portion within the elbow joint. The lesion was isointense with muscle on T1-weighted MRI, and predominantly hyperintense on T2-weighted imaging with fluid effusion in the posterior elbow joint. Post-gadolinium sequences showed thick, peripheral, and septal enhancement that was evident for not only the masses, but also the whole elbow joint. Pigmented villonodular synovitis, localized nodular synovitis, and complex ganglion were suggested as possible differential diagnoses. However, the possibility of soft tissue sarcoma could not be excluded. Incisional biopsy was performed under local anesthesia and histologic findings revealed nodular fasciitis in the elbow joint. We performed excisional biopsy under general anesthesia. After incision of the lateral capsule of the elbow, the encapsulated mass was exposed and excised. Part of the tumor existed within the distal humerus, so curettage was also performed. The lateral collateral ligament complex was repaired with nylon 4-0 suture. After surgery, a compression bandage was applied to the elbow for 2 weeks, and then active and passive range of motion exercises commenced under the supervision of hand therapists. Pathological examination proved typical histological features of nodular fasciitis, comprising cytologically bland, uniform plump spindle cells arranged in short, intersecting bundles within a collagenous stroma. Immunohistochemistry showed that the sample was diffusely positive for smooth muscle actin (SMA), but negative for S-100, desmin, CD34, anti-cytokeratin (CAM5.2), pan-cytokeratin antibody (AE1/AE3), and anaplastic lymphoma kinase. Symptoms of the elbow gradually resolved and the patient returned to normal activities after excisional biopsy. At 1 year postoperatively, plain radiography and CT revealed a large bone cyst in the distal humeral epiphysis. The lesion was hypointense to muscle on T1-weighted imaging, and predominantly hyperintense on T2-weighted fat-saturated sequences. Post-gadolinium sequences showed peripheral enhancement of the cyst. MRI revealed either a simple bone cyst or an aneurysmal bone cyst in the distal humeral epiphysis. Endoscopy-assisted curettage and artificial bone grafting (OSferion; Olympus Terumo Biomaterials, Tokyo, Japan) were performed under general anesthesia. Figure a, b shows the views from the endoscopy of the bone cyst. Pathological examination of the curettage sample showed an aneurysmal bone cyst. The blood-filled chamber was irregular in structure, with islands of bone and fibrous tissues. Immature bone formations were identified along with the fibrous septa. One year after curettage and artificial bone grafting, plain radiography showed no recurrence. On physical examination, flexion and extension of the elbow was recovered to a range of 134°–0°. Grip strength was also recovered in the affected extremity to 31 kg, compared to 30 kg on the healthy side. The patient returned to his daily activities without any symptoms.