Background/aim: Exhaled CO level provides an objective measure of a patient's smoking status. The relationship between CO levels and nicotine dependence is controversial. The aim of this study is to evaluate the relationship between exhaled CO levels and nicotine dependence as well as to demonstrate that exhaled CO levels may be used as a marker of nicotine dependence. Materials and methods: Two hundred eighty-nine patients (132 females, 157 males) were included in the study. Smoking duration, the age of smoking initiation, exhaled CO levels, and Fagerström Test for Nicotine Dependence (FTND) scores were recorded. The relationship between FTND scores and exhaled CO levels was investigated. Results: There was a statistically significant correlation between FTND score and exhaled CO levels (P < 0.001). We found that a cut-off score of 7.5 ppm for exhaled CO may be useful as a marker for heavy smoking. The sensitivity and specificity of this cut-off score for exhaled CO was 69.3% and 49.3%, respectively (P < 0.001). Conclusion: We found that exhaled CO levels significantly correlated with FTND scores. For patients who are unable to provide reliable answers to questions in the FTND, exhaled CO measurements may be used as an alternative test for estimating the status of heavy smoking.