We developed a computer-bases system to detect inappropriate use of the clinical laboratory and tested a program of physician education to reduce overutilization. We modified the hospital laboratory's computerized reporting system to identify medical patients with three or more determinations of lactic dehydrogenase (LDH) or calcium during the preceding seven days. We audited the charts of these patients, using explicit criteria, to determine whether multiple studies were justified. During the control period 51 per cent of the charts audited for multiple determinations of LDH revealed overutilization. During the study period, when physicians were notified if overutilization was found, 65 per cent of the charts showed overutilization. This difference was not significant. A simultaneous, undisclosed audit of calcium determinations also showed no change between the two periods. Therefore, this method is effective in detecting and measuring overutilization of the laboratory. It is a method which is easily adaptable to a hospital's computerized laboratory reporting system, and it can be applied without a computer. However, our program of notification and education of physicians is not effective in reducing overutilization. More effective methods of modifying physicians' use of laboratory tests need to be developed.