To investigate if age of patients, time on CAPD, episodes of peritonitis, or systemic illness (diabetes) may affect the permeability of the peritoneal membrane to small solutes, 51 patients (eight diabetic) 57.2 +/- 9.4 years of age undergoing long-term CAPD were enrolled in a prospective study of peritoneal clearances (PC), started in January 1982. The studies were repeated, when possible, every 6 months after peritonitis episodes. The results were divided according to osmolality of solutions and dwell time. The age of patients had no influence on results. Significantly positive correlations were found between PC (1.36%) of creatinine, uric acid, phosphate, and time on CAPD. Uric acid PC (3.86%) correlated directly with time on CAPD. The PC in diabetic and nondiabetic patients were similar. Patients who had more than three episodes of peritonitis showed PC similar to those observed in patients who had less than three episodes, despite a longer time on CAPD. The stability of PC in patients undergoing long-term peritoneal dialysis suggests that CAPD may permit effective dialysis over many years.