Visible changes associated with dysplasia or carcinoma have been assessed in 62 colectomy specimens from patients with ulcerative colitis. In 34 colectomy specimens with one or more carcinomas, there were 51 visible carcinomas and 65 macroscopic dysplastic lesions; only one carcinoma was found in flat mucosa. Among 28 specimens with microscopic dysplasia but no carcinoma, eight contained no visible dysplastic lesion, and the remaining 20 contained a total of 40 polypoid or elevated dysplastic areas. In 32 operative specimens removed for long-standing chronic colitis without dysplasia or carcinoma, nine contained visible inflammatory polypoid lesions. On radiological or endoscopic examination of the colon in long-standing colitis close attention should be given to the identification and biopsy of polyps, elevated nodular areas or plaques, strictures, and isolated ulcers. It is likely that colectomy specimens with visible polypoid or elevated dysplastic lesions will contain a carcinoma more often than those removed for dysplasia in flat mucosa.