OBJECTIVE To examine the effect of an intervention designed to reduce antihistamine prescribing costs in an HMO. METHODS A quasi-experimental design with clinicians at one HMO site receiving an intervention. Their prescribing behavior before and after the intervention was compared to the prescribing behavior of clinicians at other HMO sites in a non-randomized control group. METHODS A mixed model HMO in metropolitan Washington, DC. METHODS Group model clinicians at a single site received the intervention; network and group model clinicians at other sites composed the control group. METHODS An education memo addressed to clinicians discussing antihistamine prescribing costs and strategies for initial use of nonprescription or less expensive prescription agents, coupled with free antihistamine sample "trial packs" for patient use. METHODS The prescribing costs of the intervention group were compared to the prescribing costs of the non-randomized control group. The intervention group was surveyed regarding their attitudes toward the intervention. RESULTS A 2% decline in costs was noted between the baseline and intervention year for both the intervention and control groups. Though clinicians indicated they were well disposed toward the intervention, they believe patient expectations were a major obstacle to the use of cheaper agents. CONCLUSIONS We conclude that an effective intervention would require the targeting of major patient-related barriers to clinician behavior change.