PURPOSE: To determine the MR imaging findings for the differentiation of tuberculous spondylitis(TS) and pyogenic spondylitis(PS) with emphasis on the shape and MR signal characteristics of paraspinal mass. MATERIALS AND METHODS: We obtained spin-echo T1- and T2-weighted MR images of 63 surgically proved cases of TS and 15 casesof PS. Gadopentetate dimeglumine(Gd)-enhanced T1-weighted MR images in 37 cases of TS and in all 15 cases of PS were also obtained. Paraspinal mass formation, the size, shape and Gd-enhancing pattern of this mass and penetration of paraspinal ligaments by inflammatory tissue were analyzed. RESULTS: Paraspinal mass was found inboth TS(63/63) and PS(9/15). The mean longitudinal dimension of this mass was 4.3 vertebral body heights in TS and1.8 in PS. The margin of the mass was smooth in TS due to subligamentous spread of inflammation, while PS showedan irregular margin and invaded the para-aortic or para-caval space(9/15) due to penetration of inflammatory tissue through the paraspinal ligament. In the lumbar spine, however, TS showed focal penetration of the paraspinal ligament and formed psoas abscesses. After Gd enhancement, TS showed irregular thick or uniformly thinrim enhancement(35/37), suggesting caseation necrosis and cold abscess, while PS showed diffuse enhancement, suggesting abundant granulation tissue. CONCLUSION: MR imaging findings helpful for the differentiation of TS andPS were size, shape and enhancing pattern of paraspinal mass and penetration of paraspinal ligament by inflammatory tissue.