Castleman's disease is an atypical lymphoproliferative disorder of unknown origin. It has three histologic types (hyaline vascular, plasma-cell, and mixed) and two clinical types (localized and multicentric). Some systemic diseases may be accompanied with Castleman's disease, but it is unclear if Castleman's disease is relevant to rheumatoid arthritis. We experienced a case of Castleman's disease suffering from rheumatoid arthritis. A 25-year-old female with rheumatoid arthritis was referred to our hospital with a palpable mass lesion in the left upper arm. Excisional biopsy of the mass and arthroscopic synovectomy were performed, and the histological finding was compatible with hyaline-vascular type of Castleman's disease. Herein, we report this case along with brief review of relevant literature.