The purpose of this study was to evaluate the potential for online continuing medical education (CME) seminars to improve quality of care. Primary care physicians (113) participated in a randomized controlled trial to evaluate an online CME series. Physicians were randomized to view either a seminar about type 2 diabetes or a seminar about systolic heart failure. Following the seminar, physicians were presented with 4 clinical vignettes and asked to describe what tests, treatments, counseling, or referrals they would recommend. Physicians who viewed the seminars were significantly more likely to recommend guideline-consistent care to patients in the vignettes. For example, physicians who viewed the diabetes seminar were significantly more likely to order an eye exam for diabetes patients (63%) compared with physicians in the control group (27%). For some guidelines there were no group differences. These results provide early evidence of the effectiveness of online CME programs to improve physician clinical practice.