Information and communication technologies (ICT) are increasingly used in healthcare settings. Despite their technical robustness, their implementation has not always been straightforward. This is a case study of the implementation of a clinical information system for patient registration and tracking in the busy emergency department (ED) of a large English NHS University Hospitals Trust. By adopting an Actor-Network Theory (ANT) approach, the authors explore the complex intertwining of people and machines in the local setting as they negotiate the success of the project. Based on the analysis of 30 semi-structured interviews with clinical and administrative staff and, of relevant policy and project documentation, the authors demonstrate how the technologically-mediated transformation of healthcare practices is not a fixed and linear process, but the interplay of various fluctuating, performative and co-constitutive technical and social factors.