SETTING: Smoking is reported to be associated with tuberculosis (TB), but its effect on sputum smear conversion remains a matter for discussion. OBJECTIVE: To assess the effects of smoking and smoking cessation on treatment outcomes in patients newly diagnosed with TB in Iran. DESIGN: Newly diagnosed smear-positive TB patients were included in the study. All smokers participated in a smoking cessation programme. Sputum smear status was evaluated at the end of month 2, 5 and 6 of treatment, and smoking status was evaluated at the end of month 2. Differences in smear conversion rates were compared between the three groups, i.e., non-smokers, smokers and quitters. RESULTS: A total of 183 smokers and 151 non-smokers were included. When smoking cessation was assessed after 2 months, 42.6% (78/183) of the smokers were found to have quit. The cure rate at the end of 6 months, precisely compatible with the conversion rate, was significantly higher among non-smokers (P = 0.004) and quitters at 2 months (P = 0.049) than among persisting smokers (83.4%, 80.8% and 67.6%, respectively). CONCLUSION: Cure rates in patients newly diagnosed with pulmonary TB were higher in non-smokers and quitters at 2 months than in smokers. Smoking cessation advice should be included in TB control programmes.