OBJECTIVE: Evaluate the effects of long-term maintenance activities on existing portions of a large internal medicine knowledge base. DESIGN: Five physicians who were not among the original developers of the knowledge base independently updated a total of 15 QMR disease profiles; each updated submission was modified by a review of group serving as the "gold standard, " and the pre- and post-study versions of each updated disease profile were compared. MEASUREMENTS: Numbers and types of changes, defined as any difference between the original version and the final version of a disease profile; reason for each change; and bibliographic references cited by the physicians as supporting evidence. RESULTS: A total of 16% of all entries were modified by the updating process; up to 95% of the entries in a disease profile were affected. The two most common modifications were changes to the frequency of an entry, and creation of a new entry. Laboratory findings were affected much more often than were history, symptom, or physical exam findings. The dominant reason for changes was appearance of new evidence in the medical literature. The literature cited ranged from 1944 to the present. CONCLUSIONS: This study provides an evaluation of the rate of change within the QMR medical knowledge base due to long-term maintenance. The results show that this is a demanding activity that may profoundly affect certain portions of a knowledge base, and that different types of knowledge (e.g., simple laboratory vs expensive or invasive laboratory findings) are affected by the process in different ways.