Thirty-eight peritoneal biopsies from 37 patients with normal renal function or with end-stage renal failure without replacement therapy or utilizing continuous ambulatory peritoneal dialysis (CAPD), haemodialysis (HD) or a functioning transplant were examined histologically. No abnormality in peritoneal membrane morphology was observed in uraemia in the absence of dialysis. Significant abnormalities of peritoneal membrane morphology were observed in association with CAPD, the predominant finding being the development of peritoneal fibrosis which had a deleterious effect on membrane function. Abnormal peritoneal morphology was less commonly observed in patients on maintenance HD and with functioning transplants but may have implications regarding the future use of CAPD in these patients.