Measurement of exhaled carbon monoxide (ECO) has been used to confirm self-reported tobacco smoking. There is little data regarding ECO levels among Thai tobacco smokers by age. The objectives of this study were to determine ECO cutoff level to confirm tobacco smoking and to assess whether the cutoff level varies by age. During 2009 we evaluated 875 Thai volunteers aged 16-70 years, residing in Pathum Thani (central Thailand) and Khon Kaen (northeastern Thailand). Among the 875 volunteers, there were 584 non-smokers and 291 smokers. Each subject was interviewed and had their ECO level measured. The mean ECO level was 11.24 ppm among smokers and 2.25 ppm among non-smokers. The best ECO cutoff level to distinguish 291smokers from 584 non-smokers was 5 ppm (sensitivity 79.0%, specificity 89.9%).The optimal ECO cutoff level varied by age-group. For subjects aged 16-25 years, the best ECO cutoff level was 4 ppm (sensitivity 85.2%, specificity 77.5%) and for subject aged 26-70 years, the best ECO cutoff level was 5 ppm (sensitivity 79.4%, specificity 91.2%).These levels by age should be used among Thai subjects to determine smoking.