Two cases of carcinoma developing in a segment of Crohn's disease are described, one in the large intestine and one in the small intestine. From an analysis of the literature, evidence is advanced with regard to age at the time of developing carcinoma, duration of symptoms attributable to Crohn's disease, anatomical distribution, and observed incidence compared with chance incidence. It is concluded that there is a small but significant risk of carcinoma developing, both in the small and large intestines, in Crohn's disease.