Physicians rely on the medical literature as a major source of medical knowledge and data. The medical literature, however, is continually evolving and represents different sources at different levels of coverage and detail. The recent development of computerized medical knowledge bases has added a new form of information that can potentially be used to address the practicing physician's information needs. To understand how the information from various sources differs, we compared the description of a disease found in the QMR knowledge base to those found in two general internal medicine textbooks and two specialized nephrology textbooks. The study shows both differences in coverage and differences in the level of detail. Textbooks contain information about pathophysiology and therapy that is not present in the diagnostic knowledge base. The knowledge base contains a more detailed description of the associated findings, more quantitative information, and a greater number of references to peer-reviewed medical articles. The study demonstrates that computerized knowledge bases, if properly constructed, may be able to provide clinicians with a useful new source of medical knowledge that is complementary to existing sources.