Interventions to inform patients about health care options and to involve them in decisions about their care are now widely advocated. The question of which criteria should be used to judge the effectiveness of such interventions has, however, received little attention. The provision of research-based information about health care effectiveness to patients and the promotion of greater patient involvement in health care decision-making are likely to have a complex range of effects on: the information provided to patients; patients' acquisition of skills; patients' knowledge and emotions; how decisions are made; the quality of decisions; professional-patient relationships; the use of health care; the health of patients; satisfaction; and the organisation and cost of health services. Opinions about which effects are most important and how they should be measured and valued will be influenced by a variety of factors, including: the rationales and motives underlying interest in patient involvement in decision-making; the forms of patient involvement envisaged; and the types of interventions being considered. In the context of health care systems which aim primarily to improve health status and well-being, health outcomes should take priority over process variables such as decision-making behaviours and patients' knowledge.