Background. Statin Choice is a decision aid about taking statins. The optimal mode of delivering Statin Choice (or any other decision aid) in clinical practice is unknown. Methods. To investigate the effect of mode of delivery on decision aid efficacy, the authors further explored the results of a concealed 2 - 2 factorial clustered randomized trial enrolling 21 endocrinologists and 98 diabetes patients and randomizing them to 1) receive either the decision aid or pamphlet about cholesterol, and 2) have these delivered either during the office visit (by the clinician) or before the visit (by a researcher). We estimated between-group differences and their 95% confidence intervals (CI) for acceptability of information delivery (1ĝ€"7), knowledge about statins and coronary risk (0ĝ€"9), and decisional conflict about statin use (0ĝ€"100) assessed immediately after the visit. Follow-up was 99%. Results. The relative efficacy of the decision aid v. pamphlet interacted with the mode of delivery. Compared with the pamphlet, patients whose clinicians delivered the decision aid during the office visit showed significant improvements in knowledge (difference of 1.6 of 9 questions, CI 0.3, 2.8) and nonsignificant trends toward finding the decision aid more acceptable (odds ratio 3.1, CI 0.9, 11.2) and having less decisional conflict (difference of 7 of 100 points, CI -4, 18) than when a researcher delivered the decision aid just before the office visit. Conclusions. Delivery of decision aids by clinicians during the visit improves knowledge and shows a trend toward better acceptability and less decisional conflict.