A study of 113 knees of 72 patients treated with intra-articular injection of osmic acid for chronic exudative synovitis caused by various rheumatic diseases is reported. Certain cases presented abnormal radiological opacities, statistically more frequent after the injection of 200 mg than after 100 mg of osmic acid (33.9% and 15.7%). These images appear early after the injection and can remain for years. They should not be confused with intra-articular calcium deposits, or with synovial ossifications or various periosteal osteogenic processes. Their appearance does not seem to be related to the rheumatic disease treated or the therapeutic result obtained. They reflect the fixation of osmium in the synovial fatty tissue, which causes a fibrous reaction at this level. Their longterm action in the cicatrization of the synovia, or their part in a relapse of an inflammatory episode should be clarified.