Sharing of health data though the effective deployment of information systems should allow safer and more efficient health systems. However, to date many large IT system deployments in health care have had major short comings. This paper critically appraises the UK National Programme for IT and suggests where there are important lessons of for other large scale eHealth projects. Our method combined the classic evaluation methods of Donnabedian with Pawson's realistic review to analyze the impact of the program at health service, locality or major provider, and client-service impact levels. Financial incentives promoted uptake and use of IT systems at all levels. Health service level interventions that were capable of incorporation into clinical workflow were used. These included: a national unique identifier, creation of national registries and electronic transfer of data, records, and results. At the regional and major provider level we identified how vendors offer very different electronic patient record (EPR) systems which influence what is recorded and health care delivery. Using the EPR at the point of care takes longer, but this investment of time creates a more usable record and facilitates quality. National IT systems need to be clinically orientated, patient accessible, and underpinned by a secure, standardized back office system that enables messaging and information sharing between authenticated users. Learning the lessons from the UK and other large system deployments might enable other countries to leap to the forefront of health care computing.