Polyarthritis in the presence of a cutaneous T-cell lymphoma is a rare phenomenon. We describe a case of mycosis fungoides with development of a symmetric erosive polyarthritis of the small hand joints and feet, diagnosed as rheumatoid arthritis. An identical monoclonal T-cell population in the skin and in the synovium was detected by T-cell receptor gene rearrangement analysis, illustrating articular dissemination of lymphoma cells. Differentiating mycosis fungoides–associated arthritis from rheumatoid arthritis may have important implications for treatment. Based on this case, the relevant literature, and the newest disease concepts, pathogenic mechanisms and therapeutic options of mycosis fungoides–associated arthritis are discussed.