PURPOSE: To determine the extent to which magnetic resonance(MR) imaging findings can help differentiate between tuberculous arthritis (TA) and rheumatoid arthritis(RA). MATERIALS AND METHODS: This study involved sixteen patients with pathologically proven arthritis of the knee. In eight patients(mean age, 29.6 years; M:F=4:4) this was of the tuberculous variety, while eight (mean age, 47.5 years; M:F=2:6) suffered from the rheumatoid variety, which was monoarticular. For 14 patients, contrast enhancement studies were available. We retrospectively analyzed MR findings according to the demonstrated pattern of synovial thickening (regular and even, or irregular and nodular), bone erosion or abscess,bone marrow(BM) edema, the sites at which bursae were present, para-articular mass formation, and lymphadenopathy. RESULTS: In five of eight TA cases (62.5 %), irregular and nodular enhanced synovial thickening was present, while in six of eight RA cases (75%), thickening was regular and even. Bone erosions or subarticular abscesses were found in six TA cases (75%) and small erosions in three cases (37.5%) of RA. BM edema surrounding the erosion was found in four cases of TA (50%) and two of RA (25 %). In TA, edema was more extensive. In both TA and RA, all suprapatella bursae were distended while popliteal bursae were present in two cases of TA(25 %) and four of RA (50%). Para-articular masses with rim like enhancement were found in six cases of TA (75%) and in one case of RA (12.5 %). In particular, para-articular lymphadenopathy was seen in six cases of TA(75%), but not in RA. CONCLUSION: MR findings of irregular and nodular synovial thickening, extensive bone erosion, extensive BM edema, particular, para-articular abscess formation and ymphadenopathy, may help differentiate tuberculous arthritis of the knee from the rhumatoid variety.