The purpose of this study was to determine whether an observed difference in hemodialysis adequacy between states in Network 5 was due to variations in patient characteristics and to what extent dialysis center effects played a role in the observed disparity between states. This was a retrospective observational study of 6,969 patients dialyzed at centers in Maryland and Virginia. There were 3,919 patients on hemodialysis at 89 facilities in Virginia and 3,050 subjects dialyzed at 65 centers in Maryland. The mean urea reduction ratio (URR) was higher in Virginia compared with Maryland (68.2 +/- 0.1% v 66.0 +/- 0.2%, P < 0.0001, respectively), and there continued to be a mean difference in URR of 1.8% between VA and MD (P < 0.0001) after adjusting for several covariates. The differences in URR between states varied depending on facility proprietary status, size as measured by number of stations, and relationship to hospital (free-standing or hospital-based). Furthermore, the center where a patient dialyzed, when treated as a fixed effect, accounted for 15% of the variance in URR. The mean difference of 1.8% in URR between states persisted in a mixed-effects model that included all covariates along with adjusting for dialysis centers as a random effect. The disparity in dialysis adequacy between states in Network 5 could not be accounted for by demographic characteristics, case mix factors, or a large center effect observed in the region. Therefore, we conclude that underlying national reports on dialysis adequacy are heterogeneous results related to differences across regions such as states within a given Network. This difference between states is not explained by the strong center effect found on adequacy in this population of hemodialysis patients.