PROJECT SUMMARY End-stage renal disease (ESRD) patients on chronic dialysis are extremely costly to manage, accounting for 7% of Medicare spending, yet representing 1% of the population. Most of their costs stem from management of the multiple and severe comorbidities that are highly prevalent in this group, with mortality rates 20-fold higher than patients of similar age and sex in the general population. To address these issues, Medicare established in 2015 a model of specialty-oriented accountable care organizations (ACOs), known as ESRD Seamless Care Organizations (ESCOs). Constituted by dialysis centers and nephrologists, ESCOs accept responsibility for costs and quality of care of aligned beneficiaries in exchange for potential shared savings. A report on the first performance year demonstrated $75 million in savings. With promising results to date, the program is being rapidly expanded. However, much remains unclear about how these savings were achieved and what impact they will have on clinical outcomes. Not all the ESCOs achieved savings, and there was over six-fold variation in savings per beneficiary across those that did. There was also substantial variability in performance on quality measure targets, many of which focused on aspects of routine or preventive care beyond dialysis care. These variations may, therefore, be plausibly explained by the extent to which ESCOs successfully involve primary care physicians (PCPs) in the care of their patients–a decision left to the discretion of individual organizations. PCPs may be particularly effective in the management of this population, given their experience and expertise with coordination of care, their accessibility, and their generally more holistic view of the patient. Thus, an ESCO’s success at curbing spending and improving outcomes for its aligned beneficiaries may depend on its ability to increase interactions between nephrologists and PCPs. Regardless, it remains unclear whether ESCOs offer any added benefit over older primary care-oriented ACO designs like the Medicare Shared Savings Program (MSSP), in which 20% of Medicare beneficiaries on chronic dialysis currently receive their care. We propose a study with the following three Specific Aims: 1) To assess ESCO effects on interactions among nephrologists and PCPs or other specialists 2) To assess the impact that increased PCP involvement has on ESCO financial performance and clinical outcomes and 3) To compare the costs and outcomes of care for chronic dialysis patients as a function of PCP involvement in ESCOs, versus MSSP ACOs. Findings from this work will provide critical information to guide ongoing efforts at reorganization of health care for the dialysis population with the goal of improving the efficiency and quality of care delivered.