Because of the pivotal role of medical knowledge in clinical problem 'solving, it is important to understand how clinicians decide to seek additional knowledge for patient care decisions and how they choose among the resources available to them. Using a self-administered questionnaire, 126 family physicians reported their use of 11 types of knowledge resources for answering patient-specific questions arising in clinical practice. They reported almost daily use of the Physicians' Desk Reference and more often than weekly use of colleagues. There was little use reported of Index Medicus or computer-based bibliographic retrieval systems. The research literature of medicine was used infrequently and rated among the lowest of resources in terms of credibility, availability, searchability, understandability, and applicability. In deciding among a subset of knowledge resources for answering a clinical practice question, resource cost variables related to clinical availability and applicability of the information to the problem at hand appeared to be more influential in the minds of physicians than factors related to quality of the resource. These findings have important implications for the development and deployment of knowledge resources intended to be useful and used in clinical practice.