The role of the emergency nurse practitioner (ENP) has increasingly become part of mainstream healthcare delivery in major accident and emergency departments in the United Kingdom. Although some research data are available in this field, there has been little attempt to evaluate the impact of the implementation of the ENP role from the perspective of those healthcare professionals most closely involved at local level. This paper describes one part of a case study evaluation of the role in an accident and emergency department in the South Thames English region. Nine face-to-face semi-structured interviews were carried out with the key multidisciplinary stakeholders in the organization. Five major themes emerged from the data analysis: blurring role boundaries; managing uncertainty; individual variation; quality vs. quantity; and the organizational context. Whilst some professional consensus was evident regarding the benefits of the role, such as improved waiting times and patient satisfaction, there appeared also to be a degree of ambivalence, particularly regarding current role configuration, value for money, and the extent to which the role should be expanded in the future. These issues are discussed in terms of professional identity, changing role boundaries, and professional personhood. It is argued that the benefits and pitfalls of the ENP role need to be considered within the context of local service provision. The growing emphasis on clinical governance reinforces the need for ongoing audit of role effectiveness in order to meet the challenges and uncertainties of increasingly blurred professional boundaries.