Background: Poor quality of information transfer about the benefits and risks of statin drug use may result in patients not making informed decisions that they can act on in a timely fashion. Methods: The effect of a decision aid about statin drugs on treatment decision making in 98 patients with diabetes was determined in a cluster randomized trial of decision aid vs control pamphlet, with concealed allocation, blinding of participants to study goals, and adherence to the intention-to-treat principle. Twenty-one endocrinologists conducted specialty outpatient metabolic consultations. Patients in the intervention group received Statin Choice, a tailored decision aid that presents the estimated 10-year cardiovascular risk, the absolute risk reduction with use of statin drugs, and the disadvantages of using statin drugs. Patients in the control group received the institution's pamphlet about cholesterol management. We measured acceptability, knowledge about options and cardiovascular risk, and decisional conflict immediately after the visit, and adherence to pill taking was measured 3 months later. Results: Patients favored using the decision aid (odds ratio [OR], 2.8; 95% confidence interval [CI], 1.2-6.9); patients who received the decision aid (n=52) knew more (difference, 2.4 of 9 points; 95% CI, 1.5-3.3), had better estimated cardiovascular risk (OR, 22.4; 95% CI, 5.9-85.6) and potential absolute risk reduction with statin drugs (OR, 6.7; 95% CI, 2.2-19.7), and had less decisional conflict (difference, ?10.6; 95% CI, ?15.4 to ?5.9 on a 100-point scale) than did patients in the control group (n=46). Of 33 patients in the intervention group taking statin drugs at 3 months, 2 reported missing 1 dose or more in the last week compared with 6 of 29 patients in the control group taking statin drugs (OR, 3.4; 95% CI, 1.5-7.5). Conclusions: A decision aid enhanced decision making about statin drugs and may have favorably affected drug adherence. Trial Registration: clinicaltrials.gov Identifier: NCT00217061. ©2007 American Medical Association. All rights reserved.