Abstract The article presents the computed tomography (CT) findings in 15 patients with histopathologically proven primary lymphoma of the colorectum. CT is invaluable for the characterization of primary colonic lymphomas and for definition of tumor invasion and spread. The lymphomas in our series were categorized according to their CT appearance as either discrete bulky masses ( n = 6), focal mural infiltration ( n = 5), or diffuse colonic invasion ( n = 5). Primary colonic lymphomas that were manifested as mass lesions had a greater depth of mural invasion ( x = 4.3 cm) than did infiltrative lesions ( x = 2.7 cm, P <.05), and tended to present at an earlier stage. In the setting of solitary mass lesions in the colon that are not associated with adenopathy, CT findings can help to distinguish primary lymphoma from adenocarcinoma. Cecal tumors that grow into the terminal ileum, tumors which are fairly well demarcated from the surrounding pericolonic fat and show no evidence of invasion or obstruction of neighboring viscera, and tumors which perforate in the absence of desmoplastic reaction should suggest lymphoma. A clinical history of human immunodeficiency virus infection should also prompt consideration of colonic lymphoma.