A fistula between the ascending colon and the duodenum occurs not uncommonly as a complication of Crohn's disease with primarily colonic involvement. Clinical and radiological findings in four cases are presented. In three instances the fistula served as an indication for surgical intervention; after ileosigmoidostomy, ileorectal anastomosis, and ileostomy alone, the patients improved clinically but x-ray evidence of patency of the fistula persisted. Recurrent activity of the Crohn's disease in two responded to medical treatment as it did in one patient without surgery at all. The internal fistula need not be a seriuos complication of Crohn's disease and should not serve as an indication for surgical correction for its own sake.