The English National Health Service (NHS) constitutes a unique institutional context, which combines elements of hierarchy, markets and networks. This has always raised issues about competing forms of accountability. Recent policy has emphasized a move from quasi market competition towards collaboration in the form of new regional organizational arrangements known as Sustainability and Transformation Partnerships (STPs). We explore accountability relationships in STPs, focusing on the challenges of increasing horizontal accountability given existing vertical accountabilities, most notably to national regulators. We utilize a case study approach concentrated on three Clinical Commissioning Groups (CCGs) in urban and rural settings in England. We conducted in-person interviews with 22 managers from NHS organisations and local authorities and examined local documents to obtain information on governance and accountability structures. The fieldwork was undertaken between November 2017 and July 2018. We analysed results by considering which actors were accountable to what forums and the nature of the obligation (vertical or horizontal). We found that individual organisations still retained vertical accountabilities and were reluctant to be held accountable for the whole STP, given they were responsible for only part of the joint effort. Moreover, organisations did not feel accountable to STPs and instead highlighted vertical accountabilities upwards to their own boards and to national regulators; and downwards to the public. But while local commissioning organisations, CCGs engaged with their members and the public, STPs failed to engage adequately with the public. Nevertheless, there were indications that horizontal accountability was starting to develop. This could become complementary to vertical accountability by facilitating mutual learning and peer review to anticipate and defer regulatory intervention. While vertical accountability is necessary to provide oversight and apply sanctions, it is not sufficient and should be accompanied by horizontal accountability.