The knees of fifty-two patients suffering from rheumatoid arthritis (RA), 22 patients with seronegative spondylarthopathies (SA) as well as of 20 healthy volunteers were examined by magnetic resonance imaging (MRI). Osseous erosions (RA 52%-SA 18%; P less than 0.005), Baker cysts (RA 56%-SA 12%; P less than 0.005), pannus formation (RA 67%-SA 36%; P less than 0.05), and cartilage thinning with narrowing of the joint space (RA 46%-SA 18%; P less than 0.05) proved to be more frequent MRI findings in patients with RA. Additionally, in patients with RA erosions were more extensive. Follow-up MRI examinations of 19 patients revealed an improvement in MRI changes in SA within an average interval of 6 months. No substantial changes were noted in 7 of 13 RA patients. Quantitative and qualitative MRI findings of knee arthritis differ in patients with RA and SA and this was statistically significant. However, as there is considerable overlap of the MRI and radiographic changes in both groups the discriminating diagnostic value in the individual case was limited.