End-stage renal disease (ESRD) patients treated with hemodialysis have a high mortality rate, which is largely due to inadequate dialysis. Dialysis adequacy, measured by the urea reduction ratio (URR), tends to be correlated within dialysis facilities with wide variations in average center adequacy. These are characteristics of a center effect, which can have an important impact on dialysis adequacy. This study measured the center effect observed in an ESRD Network before and after a successful quality improvement project (QIP). URR values were recorded on patients sampled from 196 facilities in ESRD Network 6 before (pre-QIP, n = 5309) and after (post-QIP, n = 5753). These data was used to determine the within center correlation (rho) of individual URR values and between center variation in aggregate URR values in both samples. The overall mean URR improved from the pre- to post-QIP sample (mean URR 64.7 +/- 0.1 versus 69.8 +/- 0.1, respectively; P = 0.001). There was a high degree of within center correlation in dialysis adequacy across the facilities, which significantly diminished post-QIP (rho, 0.15 [95% CI, 0.12 to 0.18] versus rho, 0.06 [95% CI, 0.04 to 0.08]). The between center variation in mean URR also declined from the pre-QIP to the post-QIP sample (SD, 3.6 versus 2.8). In conclusion, there is a center effect on dialysis adequacy measurable in an ESRD Network, which diminishes after a successful QIP; therefore, when implementing a QIP to improve dialysis adequacy, changes in the center effect should be considered a potential indicator of the efficacy of the intervention.