We report 12 cases, complicating Crohn's disease by cancer origin. 8 patients who had chronic anorectal fistula, developed mucinous adenocarcinoma, 4 patients colorectal cancer. Features like early onset of disease, duration of disease for more than 10 years, chronic (pan-)colitis with high inflammatory activity and persistence of chronic fistulas and stenosis seem to trigger malignant transformation. Submucosal growth, overlap-syndromes with Crohn's disease and absence of sufficient clinical control of stenosis or fistula lead to advanced tumour stages at time of diagnosis combined with poor prognosis. Within four years after diagnosis 7 of 10 patients died related to metastatic disease or local recurrence. Absence of sufficient possibilities for clinical control indicates radical and early surgical resection of both endoscopically not surveilable stenosis and longstanding anorectal fistulas.