Purpose : To describe CT and MR findings of Primary nasal NK/T-cell lymphoma and to evaluate the usefulness of CT and MR imaging for initial diagnosis and during follow-up. Materials and Methods : Thirteen patients with biopsy-proven primary nasal NK/T-cell lymphoma (M:8, F:5;age 30-78, mean:47.3 years) were included in this study. CT scans were obtained in seven patients and MR images in ten, and both CT and MR images in four. During follow-up, CT images were obtained in four patients, MR images in eight, and both types of image in two. Signal intensity or attenuation, location, and shape of the tumor were assessed on CT and MR images, which were also compared in terms of evaluation of the extent of the tumor and the assessment of residual tumor during follow-up.Results : Enhanced CT scans showed slightly lower(6/7) or iso-attenuation (1/7) or the tumor than of the wall of the nasal cavity. In all cases, the tumor was of slightly lower signal intensity on T2WI and gadolinium-enhancedT1WI than nasal cavity mucosa. Infiltrative wall thickening of the nasal cavity (13/13) and a polypoid mass (5/13) were demonstrated on CT and MR images. The tumor margin was partially indistinct (6/7) or distinct (1/7) on initial CT, but distinctly identified on initial MR images in all cases. During follow-up, MR was superior to CT for the assessment of residual tumor in two patients. Conclusion : Infiltrative wall thickening of the nasal cavity may be a characteristic feature of primary nasal NK/T-cell lymphoma, and MR images were superior to CT for the assessment of tumor extent and for the diagnosis of residual tumor during follow-up.