OBJECTIVE To quantify changes in magnetic resonance imaging signals from the deep and superficial capsulo-ligamentous planes and the medial collateral ligament in flares of knee osteoarthritis. PATIENTS AND METHODS Preliminary prospective study of ten patients with medial compartment knee osteoarthritis meeting American College of Rheumatology criteria and associated with a Lequesne index of 5 or more. A grid was used to evaluate signal changes as compared to the opposite (asymptomatic) knee. Magnetic resonance images were read independently by two radiologists blinded to clinical data. RESULTS AND DISCUSSION In all ten patients the capsulo-ligamentous planes and medial collateral ligament generated low signal on T1 images and high signal on T2 images. No significant changes were seen in the asymptomatic knee. The evaluation grid produced satisfactory interobserver agreement. CONCLUSION Flares of medial compartment knee osteoarthritis are associated with changes in magnetic signal as compared to the contralateral asymptomatic osteoarthritic knee. The grid developed for this study could be used to evaluate the effects of treatment in a larger number of patients.