The Smye method has been proposed to estimate the equilibrated post-dialysis blood urea nitrogen (BUN) based on an intradialytic sample obtained approximately one hour into dialysis [1]. However, the effects of access recirculation (AR) and cardiopulmonary recirculation (CPR) on the Smye computation and the corresponding details of how blood is sampled have not been studied. The accuracy of two variations of the Smye technique was examined. In one method, the intradialytic and post-dialysis blood samples are obtained at constant blood flow. In the other, the samples are obtained after two minutes of slow flow, to determine the effects of both AR and CPR. Seventeen patients undergoing high efficiency dialysis and three- to four-hour treatment times were studied, in whom substantial AR was excluded based on slow flow urea rebound measurements during and just after dialysis. In this group equilibrated Kt/V eq(Kt/V) values computed using the Smye-derived equilibrated post-BUN estimates (constant flow samples, 1.22±0.058, slow flow samples, 1.23±0.064) were similar to eq(Kt/V) calculated from the 30-minute post-dialysis BUN specimen (1.23±0.049, P = NS). In eight other patients with severe AR (mean 35% 4.5), the accuracy of the constant flow Smye estimates was poor when the degree of AR was not constant throughout the dialysis session.