In June 1975 a clinical librarian project was initiated in the Gastroenterology Programme of McMaster University Medical Centre (MUMC). The objectives of the project were to assist patients in participating more knowledgeably in their own health care and to assist health professionals in applying the latest information from the biomedical literature to patient care. The implementation and development of the service are described, including the use of feedback from patients and health professionals to assess the impact of the librarians' participation in clinical settings. Benefits included increased accessibility of information resources to patients, a greater awareness of the usefulness of the biomedical literature and the library in patient care by health professionals, and an opportunity for the librarian to gain a first-hand view of activities and information needs in a clinical setting. Key points in determining the successful implementation of the project are given. (Author)