Of 18 patients with small-intestinal lymphoma associated with villous atrophy and crypt hyperplasia of uninvolved mucosa, most presented with acute obstruction or perforation preceded by abdominal pain and weight-loss. Malabsorption, thought to be adult coeliac disease, preceded the diagnosis of lymphoma in 5 cases. The lymphomas were composed of malignant histiocytes and were classified on morphological grounds as malignant histiocytosis. Of particular interest was the finding by immunohistochemical techniques of all major immunoglobulin classes within malignant cells of single tumors. Characterisation of this group of lymphomas as a specific entity should help in the further understanding of the relationship of malabsorption and lymphoma and in the rationalisation of treatment.