Clinical pharmacy practice needs quality safeguards and proper evaluation to ensure good performance of hospital pharmacists. We propose a method for evaluating performance using the indicator of whether pharmaceutical interventions conducted in clinical practice affect the treatment carried out by the physician. To illustrate the usefulness of this method, we report one example of the response of a physician to our clinical practice. We first designed a worksheet on which information related to pharmaceutical interventions would be recorded. The worksheet included sections for the details of the pharmaceutical intervention, whether decisions about interventions were active (pharmacists' decisions) or passive (requests from the physician), the timing of the intervention (before or after prescription), the grounds for the intervention, and whether it had any influence on the treatment, as well as a detailed record of the course of the intervention. These worksheets were used to record pharmaceutical interventions from July to December 2005. During that period, a total of 347 pharmaceutical interventions for 164 patients were recorded. Each intervention was examined from the different perspectives of type of intervention, timing (before or after prescription was issued), and reason for the intervention. Overall, it was found that 91.6% of all interventions had some influence on treatment. Because numerical results can be obtained with this method, it has the benefit of being an objective means of evaluating the contribution of pharmaceutical services. Furthermore, the validity of even those interventions that had no influence on treatment can be examined using medical and pharmaceutical findings and may improve the quality of pharmacists.