The reference method to determine the glomerular filtration rate (GFR) in children is the urinary clearance of inulin during a continuous intravenous infusion. Alternatively, the plasma clearance of inulin can be determined, which does not require urine collection. This study compared the determination of the inulin plasma clearance in 24 pediatric patients by two methods: the single injection and the continuous infusion method. In the single injection method 5000 mg/m(2) inulin was administered as bolus injection, and blood samples were drawn 10, 30, 90, and 240 min after administration. For the continuous infusion method inulin was started overnight and blood samples were collected the next day. The inulin plasma clearance determined by the single injection method was on average 9.7 ml min(-1) 1.73 m(-2) higher than the clearance determined with the continuous infusion method (95% CI: 5.3-14.2). The difference between the two methods was smaller at lower GFRs. The difference in results generated by the two methods in children is small and is considered acceptable in clinical practice. For practical reasons, the single injection method with minimum sampling is preferred.