Two patients with longstanding Crohn's disease, who developed colonic carcinoma and showed associated "precancer" identical to that seen with ulcerative colitis, are reported. The first patient developed rectal adenocarcinoma at age 42 yr, 27 yr after the onset of symptoms while the second patient had a clinically silent small colonic carcinoma at the time of colectomy, 40 yr after the onset of disease. Both patients had characteristic Crohn's disease. In both patients, in addition to the carcinoma, there was multifocal "precancer" characterized by (a) epithelial dysplasia and (b) adenomatous growth pattern and villiform proliferation of the mucosa. These changes were identical to those associated with ulcerative colitis and, in both cases, were present adjacent to and distant from the infiltrating carcinomas. It is important to note that such epithelial changes, termed "precancer" in the setting of longstanding ulcerative colitis, can also occur in association with longstanding Crohn's disease. Further studies are needed to define the incidence of such precancerous changes in Crohn's disease more clearly and to evaluate the relationship between these changes and carcinoma. Careful evaluation of patients with longstanding Crohn's disease and "precancer" is warranted to exclude concomitant carcinoma.