Primary colorectal lymphoma accounts for only about 0.2% of large intestinal malignancies. Management difficulties are compounded by a lack of consistent pathological reporting and by the use of numerous different classifications. Forty-five cases of primary colorectal lymphoma are included in this study. The presenting features are indistinguishable from those of colorectal cancer. Seven patients had a history of chronic ulcerative colitis but no other predisposing factors were identified. Immunohistochemical studies showed that all tumours were of B-cell phenotype. Most tumours were difficult to characterize using standard pathological classifications such as Kiel, except for the 11 cases of malignant (multiple) lymphomatous polyposis, which were morphologically diffuse centrocytic lymphomas. Twenty-nine lymphomas showed the morphological and immunohistochemical features associated with tumours arising in mucosa-associated lymphoid tissue. These tumours showed variable but often marked polymorphism and we have used the term polymorphic B-cell lymphoma to describe them. Consistency was achieved between three observers as to whether these tumours were low or high grade, and grade was found to be prognostically important. A modified Dukes staging system was adopted and there was a trend for early stage to give prognostic advantage. This study supports the view that surgery should be the primary treatment for localized lymphoma of the gastrointestinal tract with radiotherapy and/or chemotherapy for advanced cases and for malignant lymphomatous polyposis.