Ten cases of malignant lymphoma of the colon and rectum complicating chronic inflammatory bowel disease are presented. Seven patients had chronic ulcerative colitis with a history varying from 6 to 20 years. There was extensive colitis in six of these patients and left-sided colitis in one. All seven lymphomas showed the pathological and immunohistological features of primary B-cell tumours of the gastrointestinal tract with a predominance of high-grade tumours. Three patients had Crohn's disease of the large intestine complicated by malignant lymphoma of the sigmoid colon or rectum. The history of Crohn's disease varied from 30 months to 20 years and in each case there was fissuring and fistulae. There was extensive anal involvement in two cases. Histologically the three lymphomas were heterogeneous: one was of 'granulomatous' T-cell type and the other two were markedly polymorphic and of equivocal phenotype. They were also characterized by numerous multinucleate tumour giant cells. Primary colorectal malignant lymphoma should be regarded as a rare, but significant, complication of ulcerative colitis. Immunosuppression may be an additional factor in the genesis of intestinal lymphoma in Crohn's disease. The prognosis appears to be dependent on factors already known to be of prognostic significance in primary gut lymphomas: a predominance of high-grade tumours suggests that the outlook is generally worse than that for idiopathic primary large intestinal lymphoma.