Multiple extra-articular synovial cysts (MESC) are rarely complicated with various rheumatic diseases. We here first report a rheumatoid arthritis (RA) patient with MESC, which were extensively analyzed by a series of imaging techniques including fluorine-18-2-fluoro-D: -glucose positron emission tomography ((18)F-FDG-PET), magnetic resonance imaging (MRI), and ultrasonography. FDG uptakes in joint lesions with MESC were much higher than those reported in typical lesions of RA, suggesting that marked joint inflammation is implicated in the development of MESC.