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A four-month retrospective study was conducted to analyze the use of serum drug concentrations (SDCs) in the outpatient clinics of a teaching hospital. The general internal medicine, asthma and allergy, cardiology, and neurology clinics were selected as frequent users of SDCs. Serum theophylline, digoxin, quinidine, procainamide, carbamazepine, phenytoin, and phenobarbital concentrations obtained during patient clinic visits were audited by trained pharmacy students using medical records and computer-generated laboratory data. The SDCs were evaluated for (1) the appropriateness of how they were obtained, (2) the appropriateness of the actions taken on the results, (3) the reasons they were ordered, and (4) the patient charges secondary to obtaining SDCs incorrectly or taking inappropriate actions on the results. Overall, the SDCs from all the clinics combined were obtained appropriately 251 (61.4%) of 409 times. Appropriate actions were taken on the SDC results 82.4% of the time. A change in therapy was made after an SDC was ordered 24.4% of the time. A total of $4975 of patient charges was associated with incorrectly obtained SDCs or inappropriate actions taken on SDC results. The asthma and allergy clinic, which was staffed by a clinical pharmacist, used SDCs more appropriately than the other clinics. Pharmacists or other professionals should be used to obtain accurate SDC information and to follow-up on SDC results to maximize drug therapy and reduce costs.