Dysplasia and DNA aneuploidy in a pelvic pouch. Report of a case. A patient with an 18-year history of ulcerative colitis was operated on with colectomy, mucosal proctectomy, ileoanal anastomosis, and an S-type pelvic pouch due to intractable chronic continuous disease. The patient was followed by endoscopic controls and biopsy sampling from the pouch at regular intervals. A gradual development of severe atrophy in the ileal mucosa was followed by the development of low grade dysplasia. At the most recent endoscopic control, 4 years after the construction of the pouch, biopsies were sampled also for flow cytometric DNA analyses. DNA aneuploidy was detected in a biopsy from the center of the pouch, and a biopsy taken immediately adjacent showed low grade dysplasia. These findings underline the importance of endoscopic follow-up after construction of a pelvic pouch and focus attention to the potential of malignant transformation of the mucosa. 