48 surviving male Wistar rats, which achieved a double loop colostomy at the descending colon, developed spontaneous adeno-cardinomas at the proximal orifice of the stoma within 120-200 days. To differentiate the importance of fecal passage and eversion of the colon, the descending colon was separated from the fecal stream by a colostomy at the ascending colon, splitted longitudinally and inserted in the abdominal wall. Adeno-carcinoma arose in both positions, mainly within 120 days. Colonic mucosa, thus exposed, predominates to maligne transformation. The preferred development of cancer at colostomies, as seen systemically induced colonic cancer, is therefore explained to be of syncarcinogenic type. Similar lesions were found at human colostomies. The rareness of original carcinoma at human colostomies is caused by a more-years time of induction in relation to the high average age of patients getting a colostomy. In fact we regard this type of cancer as a biological phenomena comparable to the cancer of the gastric stump.