Implementation of the Affordable Care Act has far-reaching implications for the U.S. health care delivery system. In particular, achieving the type of integrated patient care that health care reform calls for (e.g., through “accountable care organizations” or ACOs) will require major organizational changes to the health delivery system. Despite increased focus on ACOs, little is known about how these changes will be carried out, particularly in complex urban settings where there are numerous providers and payers and long-standing networks already in place. We provide the first market-wide analysis of the potential for ACO development in this type of market. We focus on primary care physician referrals to specialists, which will play a key role in coordination of care through ACOs and will likely have to be adapted to meet quality and cost objectives. We find that physicians base referrals largely on experience and personal ties and that it would take sizable incentives (10-15% of reimbursement) for physicians to change practices. Across the market, we find that though there is uncertainty about ACO implementation, stakeholders anticipate changes in provider relationships, reimbursement, and data capacity.