The authors estimated the costs and cost savings of implementing a program of mailed practice guidelines and single-visit individual and group academic detailing interventions in a randomized controlled trial to improve the use of antihypertensive medications. Analyses took the perspective of the payer. The total costs of the mailed guideline, group detailing, and individual detailing interventions were estimated at 1000 dollars, 5500 dollars, and 7200 dollars, respectively, corresponding to changes in the average daily per person drug costs of -0.0558 dollars (95% confidence interval, -0.1365 dollars to 0.0250 dollars) in the individual detailing intervention and -0.0001 dollars (95% confidence interval, -0.0803 dollars to 0.0801 dollars) in the group detailing intervention, compared with the mailed intervention. For all patients with incident hypertension in the individual detailing arm, the annual total drug cost savings were estimated at 21,711 dollars (95% confidence interval, 53,131 dollars savings to 9709 dollars cost increase). Information on costs of academic detailing could assist with health plan decision making in developing interventions to improve prescribing.