Improving precision and economy in the prescribing of drugs is a goal whose importance has increased with the proliferation of new and potent agents and with growing economic pressures to contain health-care costs. We implemented an office-based physician education program to reduce the excessive use of three drug groups: cerebral and peripheral vasodilators, an oral cephalosporin, and propoxyphene. A four-state sample of 435 prescribers of these drugs was identified through Medicaid records and randomly assigned to one of three groups. Physicians who were offered personal educational visits by clinical pharmacists along with a series of mailed "unadvertisements" reduced their prescribing of the target drugs by 14 per cent as compared with controls (P = 0.0001). A comparable reduction in the number of dollars reimbursed for these drugs was also seen between the two groups, resulting in substantial cost savings. No such change was seen in physicians who received mailed print materials only. The effect persisted for at least nine months after the start of the intervention, and no significant increase in the use of expensive substitute drugs was found. Academically based "detailing" may represent a useful and cost-effective way to improve the quality of drug-therapy decisions and reduce unnecessary expenditures.