['To test the reliability of creatinine clearance in children on peritoneal dialysis (PD).', 'Longitudinal, case-controlled.', 'Routine clinic visits at the pediatric dialysis unit of the Universitätskinderklinik of Vienna.', 'Eleven children (2-13 years, 10-55 kg) with end-stage renal disease on PD.', "Creatinine clearance (CCr) was determined by measuring creatinine excretion (ECr) over 24 hours in both dialysate and urine. Each child had three to five separate measurements of their CCr. At the same time we also calculated the Schwartz formula clearance from the patient's height and serum creatinine, using a modified correlate.", 'Reliability of CCr was assessed by two approaches. First, we compared each serial measurement with the mean value for each patient and thereby assessed the "intramethodical" variability. Second, we compared each CCr with the simultaneous formula clearance and assessed the "intermethodical" disagreement.', 'Twenty-seven percent of the measurements of CCr were classified as unreliable based on a comparison with the mean value for each patient. Reliability was closely correlated with residual renal function (p < 0.01); only 12% of the measurements in the anuric patients were classified as unreliable (vs 31% in the patients with residual renal function). The simultaneous formula clearance was less variable than the CCr. The formula clearance had a sensitivity of 93% and a specificity of 60% for detecting unreliable values of CCr.'] Is estimation of total creatinine clearance unreliable in children on peritoneal dialysis?