A 64-year-old man suffered of long-standing joint effusions of both knees. Preceding cortisone injections had not led to a decline of effusions. Microbiological analysis of joint aspirate showed no germs. The patient was therefore referred for evaluation of a potential rheumatoid arthritis. Clinical examination confirmed significant effusions in both knees without any external inflammatory signs, other joints were not affected. Radiographs (Fig. 1, panel A) showed no osteoarthritis, but distinct periosteal proliferations (white arrows) of both distal femoral bones.