BACKGROUND: New educational programs must be developed to improve physicians' skills and effectiveness in counseling patients about smoking cessation. OBJECTIVE: To assess the efficacy of an educational program based on behavioral theory, active learning methods, and practice with standardized patients in helping patients abstain from smoking and changing physicians' counseling practices. DESIGN: Cluster randomized, controlled trial. SETTING: Two general internal medicine clinics in Switzerland. PARTICIPANTS: 35 residents and 251 consecutive smoking patients. INTERVENTION: A training program administered over two half-days, during which physicians learned to provide counseling that matched smokers' motivation to quit and practiced these skills with standardized patients acting as smokers at different stages of change. The control intervention was a didactic session on management of dyslipidemia. MEASUREMENTS: Self-reported abstinence from smoking at 1 year of follow-up, which was validated by exhaled carbon monoxide testing at one clinic; score of overall quality of counseling based on use of 14 counseling strategies; patient willingness to quit; and daily cigarette consumption. RESULTS: At 1 year of follow-up, abstinence from smoking was significantly higher in the intervention group than in the control group (13% vs. 5%; P = 0.005); this corresponded to a cluster-adjusted odds ratio of 2.8 (95% CI, 1.4 to 5.5). Residents who received the study training provided better counseling than did those who received the control training (mean score, 4.0 vs. 2.7; P = 0.002). Smokers' willingness to quit was also higher in the intervention group (94% vs. 80%; P = 0.007). A nonsignificant trend toward lower daily cigarette consumption in the intervention group was observed. CONCLUSION: A training program in smoking cessation administered to physicians that was based on behavioral theory and practice with standardized patients significantly increased the quality of physicians' counseling, smokers' motivation to quit, and rates of abstinence from smoking at 1 year.