Objective: To evaluate the effect of brief counseling on patient readiness for behavioral change and cessation/reduction of tobacco and alcohol use. Methods: This clinical trial randomized patients in blocks, stratified by risk factor. Adult smokers or at-risk drinkers undergoing surgical or diagnostic procedures were recruited. Outcome assessments and analyses were blinded. Brief counseling was compared with educational materials for the outcomes progress in stage of change and smoking/alcohol cessation/reduction. Results: Overall, 222 participants were randomly assigned to the intervention group and 218 to the control group. Among them, 28 and 18 patients were lost to follow-up, respectively. Progress in change stage was 94.1% at 1 month in both groups (RR = 1.00; 95%CI 0.95-1.05) and 94.8 vs. 90.5% at 3 months (RR = 1.05; 95%CI 0.99-1.11) in the intervention and control groups, respectively. Smoking cessation and alcohol reduction rates at 3 months were 57.2 vs. 41% (RR = 1.40; 95%CI 1.14-1.71) in the intervention and control groups, respectively. Only brief counseling led to significant differences in smoking cessation (51.4 vs. 35.1%; RR = 1.46; 95%CI 1.12-1.92). Conclusions: Brief counseling and educational materials improved patient motivation for behavioral change, but brief counseling had a greater effect on smoking cessation. Clinical trial registration: NCT03521622