We report a case of rheumatoid nodule of the right knee in a 55-year-old woman which on the basis of MR images was misdiagnosed as soft tissue sarcoma. Rheumatoid nodules occur in up to 20% of adult patients with rheumatoid arthritis and most patients with rheumatoid nodules are rheumatoid factor positive. The nod-ules are most commonly located between the skin and an underlying bony prominence. In this case, the patient had a history of rheumatoid arthritis and a soft tissue mass was present in the lateral aspect of the right knee. T1-weighted images revealed a mass of similar signal intensity to that of muscle, and on T2 weighted im-ages, a central portion of high signal intensity was noted. Enhanced T1 weighted images demonstrated periph-eral rim enhancement. The mass extended to subcutaneous tissue, with penetration of lateral patellar retinacu-lum. Excisional biopsy was performed, and the pathologic diagnosis was rheumatoid nodule. Although its radi-ologic findings are non-specific, rheumatoid nodule should - in appropriate clinical settings - be included in the differential diagnosis of a predominently solid or centrally necrotic periarticular soft tissue.