The natural logarithm formula for Kt/V proposed by Daugirdas is recognized as a valid and simple alternative to formal urea kinetic modeling (UKM) in adults receiving hemodialysis. No data have been published to validate the use of this formula in infants, children, and adolescents. We compared Kt/V derived by formal UKM with the natural logarithmic formula for 103 treatments in 21 pediatric end-stage renal disease patients receiving chronic hemodialysis. Values for Kt/V derived by formal UKM ranged from 0.65 to 2.06. Patients ranged in age from 1.8 to 22.5 years and in dry weight from 10.2 to 82.5 kg. The largest percent error between the two methods for any data point was less than 6%. The total error (absolute value percent error + 2 SD) was less than 6% across the entire range of dry weights. Our data show excellent agreement between formal UKM and the natural logarithm formula for Kt/V in pediatric hemodialysis patients, even those weighing less than 30 kg. These results support the use of the natural logarithm formula as a valid alternative to formal UKM in children. The simplicity of this formula should allow for the use of Kt/V as the best measure to study the relationship between delivered dialysis dose and outcomes in children.