PURPOSE: To evaluate MR imaging findings of joint involvement in patients with osteosarcoma MATERIALS AND METHODS: Among 166 patients with osteosarcoma treated between January 1993 and July 1998, 67(44 men and 23 women,mean age 20 years) whose tumors had invaded the epiphysis were included in this study. Those with preserved normalbone marrow signal intensity between the tumor and cortical bone were excluded. Tumors were located around theknee (n=52), the hip (n=7), the shoulder (n=5), the ankle (n=2), or the wrist (n=1). For all patients,pre-operative spin echo pre- and post-contrast enhanced MR images were obtained. In all cases, we assessed thepresence or abscence of intrasynovial mass, intraarticular disruption of cortical bone and articular cartilage,and joint effusion, and also evaluated the mass around the cruciate ligaments of the knee. All patients underwentsurgery and MR findings were correlated with the results of pathologic examinations. RESULTS: In six patients thetumor was found to involve the knee joint. Sensitivity and specificity for the intrasynovial mass (n= 6),intraarticular disruption of cortical bone and articular cartilage (n= 19), mass around the cruciate ligaments (n=7), and joint effusion (n= 12) were 83.3%, 100%, 83.3%, 33.3% and 98.4%, 78.7%, 95.6%, 83.6%, respectively, whileaccuracy for the intrasynovial mass and mass around the cruciate ligaments was 97% and 94.2% respectively. CONCLUSION: If MR imaging indicates the presence of a mass in the synovial cavity or around the cruciateligaments, this is suggestive MR findings of joint involvement.