A 13-year-old right-handdominant boy presented with pain and locking while moving the right shoulder during basketball activity and recurrent swelling around his right shoulder. There was no history of trauma, major illness, or surgery. His pain did not improve with conservative treatment, which included nonsteroidal anti-inflammatory drugs and physical therapy. On physical examination, he had a swollen right shoulder, but there were no signs of erythema, warmth, or muscle atrophy in his right shoulder. Active forward elevation of the right shoulder was 160 , external rotation was 70 , and internal rotation was at level T8. Plain radiographs of the right shoulder showed no abnormal findings (Fig. 1), but computed tomography (CT) and magnetic resonance imaging (MRI) showed several loose bodies in the axillary pouch and a large calcified mass lesion in the subscapular bursa (Fig. 2 and 3). Arthroscopy under general anesthesia in the beach chair position was performed through standard posterior and anterior portals. Arthroscopic examination showed several loose bodies in the glenohumeral joint (Fig. 4a). However, intrasynovial chondroid nodules were not seen in the glenohumeral joint. A sublabral foramen was found at the anterosuperior portion of the glenoid that communicated with the subscapular bursa (Fig. 4b). Multiple large calcified mass lesions and intrasynovial chondroid nodules were seen in the subscapular bursa, suggesting Milgram Stage 2 (transitional phase) (Fig. 4c). Loose bodies both in the glenohumeral joint and in the subscapular bursa were removed through a standard anterior portal, and proliferative thickened synovium of the subscapular bursa was also resected using a motorized shaver through the sublabral foramen. The histological examination of the loose bodies confirmed cartilaginous synovial metaplasia consistent with primary synovial chondromatosis (Fig. 5). The range of motion exercise was initiated from the day after surgery. Three weeks after the operation, he was symptom free with full range of motion and was able to return to sports. At follow-up after 5 years, he remained asymptomatic, and there was no clinical and radiographic evidence of recurrence.