Two cases of cancer of the colon have occurred recently in our series of 38 patients with exstrophy of the bladder treated by ureterosigmoidostomy. Twenty-six and 14 years elapsed between the original operation and the diagnosis of malignancy. Since 1929 the literature contains reports of 55 patients who have developed bowel neoplasms as a late complication of this form of diversion. The operation was performed for exstrophy in 35 of these, in whom the resulting tumour at the anastomotic site was malignant in 28 and benign in 7 patients. The shortest interval between ureterosigmoidostomy and recognition of the growth was 10 years with a mean latent period of 25 years. The aetiology remains uncertain but it is clear that long-term survivors after ureterosigmoidostomy should have diagnostic large bowel studies included in the follow-up.