The posthemodialysis blood urea nitrogen (BUN) concentration rebounds for 30 to 60 minutes after hemodialysis in adults. Timing of the posttreatment BUN sample has a significant impact on the calculation of Kt/V. Urea rebound and its effect on Kt/V have not been extensively studied in children and adolescents. We evaluated posthemodialysis urea rebound after 46 treatments in 18 pediatric patients with end-stage renal disease. BUN levels were drawn at 30 seconds and 5 and 15 minutes posttreatment. From these values, a logarithmic regression curve was derived that defined percentage of urea rebound (%UR) = -0.254 + 10.9*log(t), (r = 0.79). Using this equation, we estimated BUN, %UR, and Kt/V at equilibration (50 minutes) for each treatment. Estimated mean %UR was 18.7%. Single-pool Kt/V (spKt/V) and estimated double-pool Kt/V (estKt/V) values were significantly different (P < 0.0001). %UR and percentage of difference between spKt/V and estKt/V did not vary as a function of dry weight, body surface area, or K/V. To test the validity of logarithmic extrapolation, additional BUN levels were drawn at 30 seconds and every 10 minutes for 1 hour postdialysis in six patients. Percentage of difference between estKt/V and measured equilibrated Kt/V was 3.6% +/- 1.7%. Our results show %UR has a significant impact on the calculation of Kt/V in children, does not vary with patient size, and is similar to that seen in adults. We have devised an easy and accurate method to predict equilibrated BUN and calculate double-pool Kt/V in children, which requires only an additional 15-minute posttreatment BUN sample.