The pathology of 19 specimens of carcinoma complicating ulcerative colitis, resected in the University Department of Surgery of the General Infirmary at Leeds, was reviewed with particular reference to the incidence of epithelial dysplasia. The carcinomas were found to be more frequently multiple, more evenly distributed in the large bowel, and much more often of atypical macroscopic appearances and of mucoid histological type than ordinary colorectal carcinomas, but the proportion of poorly differentiated tumours complicating ulcerative colitis was not as high as previously reported. Of the patients in our series 26% are alive and well at least 5 years after surgery. Unequivocal epithelial dysplasia was demonstrated in some part of the large intestine in 15 of 19 specimens with colitis carcinoma, but was also found in 4 of 14 specimens from a "control" series of patients with longstanding total colitis but without carcinoma. Clearly, therefore, the finding of dysplasia in a rectal biopsy of a patient with colitis is not a reliable guide to the presence of a frank carcinoma elsewhere in the bowel. Whether it indicates a special predisposition to the development of a growth in the future, as might be postulated from the analogy of similar changes in other organs, cannot be determined on the data of this study. The fact that epithelial dysplasia when present in colitis is often patchy in distribution and frequently spares the rectum even in patients with definite carcinomas makes a negative rectal biopsy particularly unreliable in deciding on the absence of a tumour or the lack of predisposition to it. Multiple biopsies from different parts of the colon as well as the rectum would thus seem to be desirable if mucosal sampling is to be employed as a screening test.