Two cases of adenocarcinoma arising in sequestrated duplications of the bowel in adults are reported. The literature is reviewed and a tendency for duplications to present in early childhood is noted. Although relatively few of these anomalies persist into adult life 8 cases of neoplastic change have now been reported. The histological features of many of the duplications undergoing malignant degeneration suggest the presence of underlying epithelial instability in this condition. It is postulated that duplications of the large bowel have malignant potential.