Medical information needs were examined in a survey of sixty-seven physicians selected from the faculty and housestaff at Johns Hopkins Hospital. A standardized questionnaire was administered personally by a medical informatics physician to collect data on information needs, attitudes, and previous experience with computers. The types of information most frequently required by both faculty and housestaff were treatment recommendations and differential diagnosis. The sources of reference information most commonly used were textbooks and colleagues. The information needs of housestaff differed significantly in several categories from those of faculty physicians. Housestaff more frequently needed information for patient care (P less than 0.05), and preferred the use of textbooks (P less than 0.01) and handbooks (P less than 0.001) as information sources. Faculty more frequently needed information for activities unrelated to patient care (P less than 0.01) and placed greater importance on basic science information (P less than 0.01). When asked to suggest references for online access, the respondents named 143 journals and textbooks, with journals overwhelmingly preferred over textbooks. Only one reference, the New England Journal of Medicine, was requested by a majority of the respondents. The importance of a broad understanding of physician information needs through improved data-collection techniques is discussed as a means of increasing the use of medical information systems.