In performing colonoscopy over a 4-yr period on 112 patients with long-standing ulcerative colitis, a dysplasia-associated lesion or mass (DALM) was found in 12. This appeared as either a single polypoid mass (5 cases), a plaquelike lesion (2 cases), or multiple polyps (5 cases). In 7 of these 12 cases carcinoma was subsequently found. This was particularly true in the 5 cases of single polypoid masses where all 5 contained invasive carcinoma. In none of the cases associated with malignancy did multiple biopsies of the DALM reveal invasive carcinoma. In only 2 of the 7 patients with carcinoma was the dysplasia "severe," being "mild" or "moderate" in the remaining 5. In the 27 patients with dysplasia found incidentally in random biopsies of flat mucosa only one carcinoma was found, in contrast to the 7 cancers in 12 patients with DALM. This difference leads us to regard the finding of DALM, especially a single polypoid mass, as highly significant in patients with long-standing ulcerative colitis. It carries a sufficiently high risk of cancer, thereby constituting a strong indication for colectomy.