That a treatment selected for a given condition works, or that it works better than alternative treatments, or that it was selected because it works as well as but is cheaper than alternative treatments, should be of pivotal concern to clinicians and is of central concern to patients and to health care managers. Attempts to address these concerns have resulted in what is now widely termed the 'effectiveness movement'. The protagonists of the movement have been concerned to create a culture of evaluation and inquiry within which the formulation of evidence-based clinical guidelines and their introduction into routine practice have played a prominent part. The need to ensure cost effectiveness of clinical intervention has been at least as emphasized as the need to ensure the clinical effectiveness of health care interventions. Although cost-effectiveness analyses are now an indispensable feature of practice guideline formulation and treatment evaluation, few studies have examined any deterioration in patient outcome associated with successful cost containment. An adequate understanding of the concept of clinical effectiveness and the associated aims of the 'effectiveness movement' is central to an understanding of the future nature and extent of health service provision, not simply in the UK but also internationally. Having examined the concepts of efficiency and appropriateness previously (O'Neill, Miles & Polychronis 1996, Journal of Evaluation in Clinical Practice 2, 13-27) we move in this second of two articles to a detailed explanation of the concept of effectiveness, and to an examination of the derivation and use of clinical practice guideline, concluding with a consideration of the role of practice guidelines in ensuring the cost effectiveness of health care intervention. The reservation is expressed that a 'guidelines culture', when established, will be manipulated by health care commissioners for largely political purposes, creating a systematic bias in the purchasing process that will actively disadvantage a range of patient groups.