Mucosa-associated lymphoid tissue (MALT) lymphoma is a low-grade lymphoma that differs from high-grade non-Hodgkin lymphoma both clinically and histologically. The CT appearances of MALT lymphoma are described. Of 40 patients referred with biopsy-proven MALT lymphoma, only seven had not had gastrectomy or chemotherapy prior to CT examination. The CT scans of these seven cases were analysed for the degree and extent of gastric wall thickening, enlargement of abdominal and extra-abdominal lymph nodes, and presence of extra-nodal disease. In all patients the stomach was distended with oral contrast medium and scans performed at narrow collimation, after intravenous administration of 20 mg hyoscine butylbromide. In six patients focal thickening of the gastric wall was 1 cm or less. One patient had thickening of over 4 cm. There was no enlargement of abdominal or extra-abdominal lymph nodes or extension to adjacent organs. Thus on CT, at presentation, MALT lymphoma results in minimal gastric wall thickening, unlike high-grade non-Hodgkin lymphoma, which typically causes bulky gastric disease, nodal enlargement and extension into adjacent organs. CT is therefore of limited value in monitoring response to treatment. With disease greater than minimal thickening, transformation to a higher grade should be considered.