The use of antibiotic prophylaxis for unnecessarily prolonged periods after surgical procedures can contribute to increased health care costs and adverse drug reactions as well as the development of antibiotic-resistant infections. Hospitals are under economic pressures to develop methods to control the excessive use of these drugs. We expanded the capabilities of our hospital information system to monitor the duration of surgical antibiotic prophylaxis. For six months during one year we used the computer system to monitor antibiotics received by every surgical patient and to identify patients receiving antibiotic prophylaxis longer than was deemed necessary according to generally accepted guidelines. For six months in the following year we used the system to monitor and identify the same types of patients and clinical pharmacists placed antibiotic "stop orders" in the charts of the patients identified by the computer. Surgical patients received an average of 19 doses of antibiotics in the first year compared with 13 doses in the second year (p less than 0.001). The average cost of antibiotics received more than 48 hours after the operation was $42 less per patient in year 2 than in year 1, resulting in a potential cost savings of $44,562 in six months. The computer system was found to be an efficient tool for monitoring all antibiotics given to surgical patients and identifying patients receiving antibiotic prophylaxis longer than necessary. Clinical use of this system appears to have resulted in improved usage of antibiotic prophylaxis.