Pressure to utilize research evidence in decisions about patient care and population health, so-called evidence-based medicine, has swept the health care systems of most industrial countries. In the UK, the attention of policy makers has recently turned from the production of more 'effectiveness information' to the more fundamental challenge of understanding the factors involved in influencing the attitudes and practices of health care professionals. This article seeks to contribute to this process by reporting the findings of an evaluation of a clinical effectiveness initiative established in Wales between 1996-9 (Locock et al. 1999). The study shows that a number of different factors are involved in improving the implementation of effectiveness information, including the strength of the evidence, and the role of peer influence. However it is the interaction of various elements rather than any single factor, which appears to be crucial. These findings are important not only for health but for informing future interventions across other parts of the public sector. © 2002 Taylor and Francis Ltd.