The quality of diagnosis-guiding information available from textbooks, journal articles and clinical specialists is compared. The diseases, systemic lupus erythematous (SLE), ankylosing spondylitis (ANK) and herpes encephalitis (HE), were chosen to test this quality. Three knowledge files were created for both SLE and ANK and one knowledge file was created for HE. Two of the knowledge files for SLE and ANK utilized information from a textbook. The third knowledge file for SLE and ANK and the single HE file were a compilation of all available information. Case reports, three of SLE, three of ANK, and two of HE, were collected and presented to a computer program able to generate a differential diagnosis. All the knowledge files associated with the diagnosis of a case report were differentiated from those associated with other diagnoses. Secondary diagnoses included a compiled knowledge file in four and a textbook derived file in two of eight cases.