We measured postdialysis urea rebound (PDUR) 30 minutes after dialysis in 92 chronic hemodialysis patients. The impact of PDUR on the estimation of dialysis delivery assessed by urea reduction ratio and Kt/V was evaluated. Total recirculation, access plus cardiopulmonary, was measured at the end of dialysis with the two-needle low blood flow method. The mean age of the 92 patients (49 men and 43 women) was 59.6 +/- 1.4 years. Thirty-eight patients had been receiving erythropoietin therapy for more than 3 months. Fifteen patients had central venovenous access and 77 had peripheral arteriovenous access. Sixty-five patients were dialyzed using hemophan membranes and 27 were dialyzed using polyacrylonitrile membranes. The mean blood flow rate was 240 +/- 28 mL/min and the mean length of the hemodialysis sessions was 3.6 +/- 0.1 hours. Kt/V was calculated with Daugirdas' second-generation formula. The mean PDUR was 16.6% +/- 0.8% (range, 2% to 44%) (n = 92), and significantly decreased the mean urea reduction ratio from 61.7% +/- 0.8% to 55.5% +/- 0.9%, the mean Kt/V from 1.14 +/- 0.03 to 0.97 +/- 0.02, and the mean protein catabolic rate from 1.06 +/- 0.04 to 0.98 +/- 0.02 (P = 0.0001). The effective Kt/V at 30 minutes postdialysis was well predicted by using a recently proposed equation: eKt/V30 = Kt/Vsp - (0.6 x Kt/Vsp/t) + 0.03, with a mean value corresponding also to 0.97 +/- 0.02. However, this estimation was less predictive in patients with very high PDUR. Moreover, PDUR showed only a weak negative correlation with dialysis session length (r = -0.28) and predialysis patient weight (r = -0.29), and showed no correlation with predialysis serum urea level or with blood flow rate. However, dialysis efficiency, as assessed by K/V, presented a correlation of 0.54 with both PDUR and the difference in Kt/V when using urea immediately postdialysis and at 30 minutes. The mean total recirculation was 7.4% +/- 0.6% (n = 86). Postdialysis urea rebound, calculated between 30 or 120 seconds and 30 minutes after dialysis to deduce the influence of recirculations, was reduced but remained important with a mean of 11.8% +/- 0.7%. Thus, total recirculation contributed to nearly 30% of PDUR. The 24 patients with PDUR > or = 20% were compared with the 68 patients with PDUR lower than 20%: women and patients with higher K/V and higher total recirculation presented greater PDUR. Because of relatively few predictive factors for PDUR, its potential considerable impact on dialysis delivery estimation, and the influence of recirculations on the total PDUR amount, total recirculation and PDUR should be determined on an individual basis in chronic hemodialysis patients. The equation proposed to estimate effective Kt/V at 30 minutes is accurate in most patients with PDUR lower than 30% and is a simple alternative.