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The patient, a 66-year-old woman, presented with a mass in her upper arm that had developed 2 weeks previously and was growing. She had a 39-year history of rheumatoid arthritis (RA). Seven months earlier, AA amyloidosis in multiple organs, including the intestine and liver, had been diagnosed, and tocilizumab therapy had been initiated. The tocilizumab treatment was effective for the amyloidosis, and RA disease activity was low. Physical examination on presentation revealed 4 cm of an elastically hard mass without tenderness or heat on the left triceps brachii muscle (A). Magnetic resonance imaging (MRI) demonstrated a 33-mm protrusion of the fluid-filled cystic lesion between the lateral head and the long head of the triceps brachii muscle (B). This cystic lesion was continuous with the synovial cavity of the left subacromial bursa, and aspirated fluid contained large amounts of inflammatory cells, including neutrophils, monocytes, and lymphocytes. Amyloid arthropathy sometimes involves the shoulder joints and causes bursitis. However, the characteristic finding of amyloid deposition in the shoulder (Konisiike T, Hashizume H, Nishida K, Inoue H, Nagoshi M. Shoulder pain in long-term haemodialysis patients: a clinical study of 166 patients. J Bone Joint Surg 1996;78:601–5) was not observed in this patient. Therefore, we concluded that the cystic lesion was an atypical synovial bursa due to RA. MRI also demonstrated the accumulation of a large amount of synovial fluid in the shoulder joint cavity and the subacromial bursa, with long-term inflammation in the shoulder joint having caused the synovial bursa. Synovial bursa in the shoulder joint usually develops in front of the bicipital groove or on the lateral side of the subscapular area. In this patient, the accumulation of large amounts of synovial fluid and the atrophy of the triceps brachii muscle may have caused the protrusion of the synovial bursa through the triceps brachii muscle. The findings in this patient suggest that atypical bursa should be considered in the differential diagnosis of subcutaneous mass in patients with RA and sarcopenia.