OBJECTIVE: Literature suggests that 'Academic Detailing' is a useful 'evidence-based' intervention in promoting rational drug therapy by encouraging appropriate use of cost-effective generic pharmaceuticals. This study examines possible impact of academic detailing on trends in change of proportions of generic drug prescriptions among ambulatory care physicians serving patients covered by a state health insurance program. METHODS: The study was conducted using retrospective data available from the pharmacy benefit management company serving the insurance program. The target physician population comprised of two experimental groups (Charleston, n = 251; Morgantown, n = 214) - the top 30th percentile of all the physicians in the areas chosen based on prescribing volume and average prescription cost. University-trained academic detailers visited them once every month and also provided them with educational materials. A 'comparison' group (n = 359) was chosen similarly but was not visited by a detailer at any time. Monthly generic prescribing percent were determined for all three groups for a period of 12 months before the intervention and 6 months after the intervention. Two therapeutic classes with ample generic choices - antibiotics and anti-hypertensives, were studied. RESULTS: In case of antibiotic prescriptions, while mean increase in percent generic prescriptions went up in both the experimental groups (0.26 to 2.41 in Charleston; -0.18 to 2.35 in Morgantown), mean change in percentage generic prescriptions reduced further (-0.6 to - 3.36) in the comparison group. Though over a much shorter intervention period - similar trends were observed with anti-hypertensive prescriptions. In Charleston, there was a sustained (mean rate of change over intervention period =1.37) trend in increase in proportion of generic prescriptions while the proportion declined in the comparison group (mean rate of change over intervention period = -0.44). CONCLUSIONS: Academic detailing appears to be a promising strategy for maintaining or increasing generic prescribing by physicians in ambulatory settings.