Pessimism has been expressed in the medical literature as to the efficacy of educational interventions in modifying practice patterns of graduate physicians. As a result, a prospective controlled trial of a specific form of educational intervention, the physician tutorial, was designed to test this belief. Physicians in the experimental group were surveyed to assess their knowledge of the effectiveness, cost and side effects of antibiotics used in the treatment of a preselected index condition, and a tutorial was developed to modify suboptimal prescribing patterns. Antibiotic usage patterns were initially similar for the experimental and control physicians. Prescribing patterns afterwards were statistically different, with the experimental physicians increasing their prescription of the encouraged antibiotics and decreasing that of the discouraged antibiotics. This was accompanied by a statistically significant decrease in direct drug charges. These results suggest that educational programs can be effective in modifying graduate physician prescribing practices.