PURPOSE: To evaluate the characteristic MR findings of hemophilic arthropathy of the knee. MATERIALS & METHOD: Seven knees in six patients with hemophilia (five hemophilia A and one hemophilia B) were retrospectively studied with MR images and plain radiographs. Patients were aged between 2 and 20 years (mean, 11) and all had a clinical history of repeated hemarthrosis. MR images of the knee were analyzed with respect to intra- andextra-articular hemorrhage, the state of synovial tissue, articular cartilage, bone, menisci, and ligaments. RESULTS: Synovial hypertrophy and articular cartilage destruction were revealed in all seven knees ; pannus wasfound in four, and was seen as low signal intensity on T1-weighted image and high signal intensity on T2-weighted images. All five instances of synovial hypertrophy and pannus were enhanced. Joint effusion, presented in five ofseven knees, demonstrated slightly low signal intensity on T1-weighted image and high signal intensity on T2-weighted images, and was associated with peripheral low signal intensity of hemosiderin. Subchondral and marginal erosion was seen in six cases, patellar deformity in three, meniscal damage in four and cruciate ligament damage in one case. CONCLUSION: MR is superior to plain radiography in demonstrating chronic repeated hemarthrosis (manifested as thick intra-articular effusion), hemosiderin, synovial hypertrophy, erosion ordestruction of articular cartilage and bone, and meniscal or cruciate ligament injury of hemophilic arthropathy ofthe knee. MR is therefore thought to be a useful imaging study for accurate evaluation of hemophilic arthropathyof the knee.