PURPOSE: To evaluate MR characteristics of periosteal reactions and subperiosteal abscesses in osteomyelitis as compared with radiographs. MATERIALS AND METHODS: We retrospectively reviewed 28 patients(18 males, 10 females) with osteomyelitis. Nineteen patients underwent MR imaging with 0.5 T. We analyzed for morphologic patterns, intervals of appearance and disappearance of periosteal reactions after symptom onset. Twenty-three patients were confirmed by surgery and 5 patients by the radiologic findings, clinical and laboratory data. RESULTS: Periosteal reaction appeared as low signal intensity arc on the T1- and T2-weighted axial images. In 3-7 days after symptom onset, periosteal reactions and subperiosteal abscesses were noted on MR imaging, and only two of eleven patients were noted in radiographs. Periosteal reactions of 8-14 days after symptom onset were demonstrated in all of 13 patient on MR, and 9 on plain radiographs. All patients with 2 weeks after symptom onset showed perioste.al reaction in plain film and MR. The periosteal reactions persisted until approximately 2 to 3 months after treatment on follow up radiographs. CONCLUSION: Periosteal reactions in osteomyelitis are detected on the MR imaging earlier than plain film. MR is valuable in detecting subperiosteal abscess which is not appecent in simple radiographs. Periosteal reactions can be seen on MR in 3 days after symptom onset and persist for 2-3 months after treatment.