This study took place in the UK, National Health Service (NHS), against a background of poor staff morale, continuous lack of funding and a perverse management performance measurement system. The study investigated the impact of Theory of Constraints (TOC), a change methodology previously employed in the private sector and now adapted to the health sector, on three NHS Trust departments, Neurosurgery, Eyes and ENT, especially in relation to reducing waiting lists in the system and improving throughput of patients. Data were collected over a period of forty months, on a number of NHS performance indicators, before and after the TOC intervention. An interrupted time series design with switching replications (Cook and Campbell, 1979) was used to investigate the impact of the intervention. An overall ARIMA analysis indicated that TOC had an impact in both Eyes and ENT. 16 out of 18 measures went in the direction of the hypotheses, the probability of these changes in the predicted direction by chance alone was 0.0006. However, there was a lack of significant improvements in Neurosurgery that was associated with the size of the system, complexity of treating neurological disorder, heavy reliance on support services, impact of emergencies on elective work and the motivation and receptiveness of staff to the proposed changes. In order for organisations to maximise the benefits of TOe organisations should take into account the social environment in which they exist. The importance of customising the intervention to the local need of each department, leadership requirements and robust project management, as failing to do so can potentially derail the change process.