The traditional style of medical decision-making in which doctors take sole responsibility for treatment decisions is being challenged. Attempts are being made to promote shared decision-making in which patients are given the opportunity to express their values and preferences and to participate in decisions about their care. Critics of shared decision-making argue that most patients do not want to participate in decisions; that revealing the uncertainties inherent in medical care could be harmful; that it is not feasible to provide information about the potential risks and benefits of all treatment options; and that increasing patient involvement in decision-making will lead to greater demand for unnecessary, costly or harmful procedures which could undermine the equitable allocation of health care resources. This article examines the evidence for and against these claims. There is considerable evidence that patients want more information and greater involvement, although knowledge about the circumstances in which shared decision-making should be encouraged, and the effects of doing so, is sparse. There is an urgent need for more research into patients' information needs and preferences and for the development and evaluation of decision-support mechanisms to enable patients to become informed participants in treatment decisions.