The term “intraosseous synovial cyst” is used to designate both the epiphyseal cyst-like lesions seen in patients with rheumatoid arthritis (RA) and mucoid cysts, which occur in a different setting. We report the case of a patient in whom a 4-cm cyst-like lesion developed in the left tibia 18 years after onset of RA and 6 years after osmic acid synovectomy of the left knee. Positive contrast arthrography and magnetic resonance imaging visualized a communication between the lesion and the joint space. Preexisting bone and joint lesions and increased intraarticular pressure play a major role in the genesis of cyst-like lesions in RA. In our patient, the osmic acid synovectomy may have contributed to the development of the lesion. “Synovial cyst” is a misnomer for these giant lesions, which are geodes rather than cysts. Despite their low incidence, these lesions deserve attention because they raise diagnostic and therapeutic problems.