In these times of economic constraint, libraries of all types are under increasing pressure to evaluate their services. Hospital libraries face a particular challenge because the goals of the health care system demand that the relevance of library services to patient care be determined. The hospital librarians in Rochester, New York, responded to this challenge by developing a research project that explored the impact of library services on clinical decision making. A systematically sampled group of 448 physicians in the Rochester area agreed to participate in the study between September 1990 and March 1991. The physicians were asked to request some information related to a current clinical case and then to evaluate its impact on the care of their patients. Senior medical staff or administrators acted as study facilitators in each of the fifteen participating hospitals. As a result of the information provided by the library, 80% of the 208 physicians who returned their questionnaires said that they probably or definitely handled some aspect of patient care differently than they would have handled it otherwise. Changes in the following specific aspects of care were reported by the physicians: diagnosis (29%), choice of tests (51%), choice of drugs (45%), reduced length of hospital stay (19%), and advice given to the patient (72%). Physicians also said that the information provided by the library contributed to their ability to avoid the following: hospital admission (12%), patient mortality (19%), hospital-acquired infection (8%), surgery (21%), and additional tests or procedures (49%). The physicians rated the information provided by the library more highly than that provided by other information sources such as diagnostic imaging, lab tests, and discussions with colleagues. In addition to confirming earlier research findings that information provided by hospital libraries is perceived by physicians as having a significant impact on clinical decision making, the results increase our store of scientific knowledge about the specific nature and extent of the impact of information provided by the hospital library.