The purpose of this study was to determine whether patients with end stage renal disease treated with hemodialysis were correlated in dialysis adequacy within facilities. This was a retrospective analysis of dialysis adequacy based on urea reduction ratio (URR) values from 6969 patients dialyzed at 154 facilities. The within-center correlation was quantified using the between-center variation and the parameter p that was derived using ANOVA tables and mixed effects models. The variation in center means for URR was wider than expected for independent observations (52.9-76.1 versus 60.7-73.8, respectively). Furthermore, there was a significant within-center correlation in URR values across all facilities (p = 0.136, P<0.0001), which persisted after adjusting for patient specific covariates, facility characteristics, and state. In conclusion, there was a substantial within-center correlation in dialysis adequacy that reflected important center effects on the outcome of ESRD patients.