The clinical and financial impact of an aminoglycoside monitoring service was determined. All patients admitted to a 74-bed general medicine unit and treated with tobramycin or gentamicin during a six-month study period were eligible for the study. The first three months served as a control period during which pharmacists used published audit criteria and modifications of those criteria to monitor the appropriateness of gentamicin and tobramycin use in patients but did not attempt to intervene in aminoglycoside prescribing. During the next three months, pharmacists provided physicians with recommendations for choice of drug, coordinated blood sampling times, and designed individualized dosage regimens for all patients treated with gentamicin or tobramycin. Data for financial analysis were obtained from pharmacy profiles and medical records, and the cost:benefit ratio for the service was calculated. A total of 118 patients were included in the study. Significant improvements in appropriateness of tobramycin therapy, adequacy of loading dose, frequency of monitoring for ototoxicity, and serum concentration monitoring were noted in the intervention group. Despite an increase in gentamicin use from 20% in the control group to 61% in the intervention group, the incidence of aminoglycoside toxicity did not increase significantly. The cost:benefit ratio was 1.13, which indicates that the service is an appropriate use of resources. The aminoglycoside monitoring service had a favorable impact on the use and cost of aminoglycoside antibiotics. Expansion of the service to all areas of the hospital served by satellite pharmacies could reduce drug expenditures by as much as $55,000 per year.