Antibiotics are a medication class for which inappropriate prescribing is frequently described. This study examines the effectiveness of a mailed intervention combining confidential prescribing feedback with targeted educational bulletins in increasing the use of less expensive, first-line antibiotics by practising physicians. The participants were 251 randomly selected primary care physicians from southern Ontario who consented to participate (135 in the feedback group and 116 in the control group). Prescribing data were obtained from the claims database of the Ontario Drug Benefit program, which covers all Ontarians over age 65 years for drugs selected from a minimally restrictive formulary. Confidentially prepared profiles of antibiotic prescriptions coupled with guidelines-based educational bulletins were mailed to the intervention group every 2 months for 6 months. The control group received no intervention until after completion of the study. The main outcome measures were change from baseline in physician's median antibiotic cost and the proportion of episodes of care in which a prespecified first-line antibiotic was used first. The median prescription cost of about $11 remained constant in the feedback group but rose in the control group (change of $0.05 v. $3.37, p < 0.002). First- line drug use increased in the feedback group but decreased in the control group (change of 2.6% v. -1.7%, p < 0.01). In a mailed survey of 100 feedback recipients (response rate 76%), 82% indicated that they would participate readily in another, similar program. A simple program of confidential feedback and educational materials blunted cost increases, increased the use of first-line antibiotics and was highly acceptable to Ontario primary care physicians.