While patient participation in treatment decisions is increasingly advocated in medical literature, patient demand has been considered to cause unnecessary prescribing. Using the concept of customer participation as discussed in services marketing and management literature as a theoretical base, the paper analyses the influence of patient participation on the medical service process and treatment decision-making. A qualitative, explorative study was conducted to investigate American and British physicians' views on patient participation in the treatment of osteoporosis and schizophrenia. It became evident that in the cases of both osteoporosis and schizophrenia, patients influence prescribing decisions despite the significant difference in their willingness and ability to participate. The manifestations of patient participation were divided into three groups: (1) resources, such as the patient's condition and information about it, and his/her preconceived notions and preferences, (2) actions, such as preparing for the service, negotiating decisions and implementing the treatment, and (3) the patient's role expectations and inclination to participate. The influence of such manifestations on prescribing decision-making is discussed in detail, and differences between the studied illnesses are explained. Implications to health-care managers and practitioners are discussed.