This study evaluated a practice development programme consisting of nine projects together known as STEP (South Thames Evidence-Based Practice Project). The aim of STEP was to establish and assess evidence-based practice in nursing and other health care practice areas. Objectives of the independent evaluation were to identify and assess outcomes from the process of change and investigate the association between these intermediate outcomes and patient outcomes. Outcomes were measured before and after the changes were introduced. Data collection methods included interviews with the change agents and other stakeholders, and a questionnaire to staff in each centre. Patient outcome data were collected from each centre. The findings revealed 'dissemination' of information to staff and 'adherence' by staff to new practice guidelines to be important intermediate outcomes in the process of change. The need emerged for a supportive organizational culture and commitment, recognition of the importance of change and a credible change agent. There was some evidence of linearity in the process of change in that a logical route appeared from dissemination of information to staff through adherence to the change guidelines to improvement in patient outcome in six of the nine centres. Linearity was less apparent in the other three centres, where the process of change seemed more dynamic and chaotic. These three centres were affected more than the others by organizational barriers to change. Our conclusions support the view that the linear model of change can work in settings with high levels of certainty but complexity theory is more likely to underpin the process of change in organizations characterized by uncertainty.