The accuracy of methods for measurement of creatinine in plasma, urine and dialysate is of great importance in continuous ambulatory peritoneal dialysis (CAPD) patients, to assess the adequacy of CAPD (creatinine clearance) and to monitor the nutritional status (creatinine kinetic lean body mass). The methods most widely employed for creatinine determination are Jaffe's reaction and the enzymatic method, however these techniques may suffer from glucose interference, particularly for dialysate. We compared creatinine values obtained by Jaffe's reaction, the enzymatic method and high pressure liquid chromatography (HPLC) for three creatinine calibration curves prepared in three dialysis solutions with various concentrations of glucose and for plasma, urine and dialysate of 40 CAPD patients. High values of intercept of creatinine calibration curves were observed only with Jaffe's reaction and the enzymatic method in dialysis solutions. In plasma, urine and dialysate, creatinine values obtained by HPLC were always found to be lower than those measured by the other two methods. Concerning creatinine measurement in plasma and urine, Jaffe's reaction and the enzymatic method appeared equivalent. However it must be noted that, in dialysates, the enzymatic method may have glucose interference, and the use of a correcting factor for glucose with Jaffe's reaction is convenient. Nevertheless HPLC remains a method of reference. It is concluded that, for the CAPD patient, follow-up by creatinine kinetic lean body mass or creatinine clearance is possible provided that the same creatinine assay method is used in all biological fluids.