Colonic epithelial mucin was investigated histochemically in biopsy specimens from a group of patients who had undergone ureterosigmoidostomy. For comparison, colonic biopsy samples from uninvolved mucosa adjacent to carcinomas from another group of patients and from a group of patients undergoing sigmoidoscopy for hemorrhoids were also studied. The high-iron diamine-alcian blue (HID-AB; pH 2.5) method was used, and the proportions of HID-positive and AB-positive cells were assessed semiquantitatively. In both ureterosigmoidostomy and cancer groups, highly significant increases in the proportions of AB-positive cells (sialomucins) were observed, particularly in the middle and lower segments of the crypts. Ureterosigmoidostomy introduces a high risk for the development of colonic carcinoma. Morphologic features that could account for such a high risk were investigated, and an abnormal pattern of colonic mucin secretion after ureterosigmoidostomy was demonstrated. Although this abnormality cannot be related specifically to ureterosigmoidostomy, data from the present investigation suggest that histochemical studies of colonic specimens from patients who have undergone ureterosigmoidostomy may provide a useful tool for follow-up studies.