The aim of this study was to show the interactions between nicotine dependency, exhaled carbon monoxide (CO) and smoking status including environmental passive tobacco smoke exposure in a sample of Turkish adolescents. This study was a cross-sectional research conducted among high school students of Erzurum province (n=536). The level of exhaled CO of all participants who accepted to participate in the study was measured by Microbio CO Analyzer (Microbio Med). Nicotine dependency was measured by a six-item version of the Fagerstrom Tolerance Questionnaire. Mean age of the adolescents was 17.0 +/- 1.6 (median: 17) and 77.1% were male. Of the study population, 30.2% and 11.4% of the students were regular and occasional smokers, respectively. Non-smokers who were exposed to environmental tobacco smoke had significantly (p<0.01) higher exhaled CO levels (2.8 +/- 2.6 ppm) than not-exposed non- smokers (1.8 +/- 1.9 ppm). Only 8.6% of adolescents who reported themselves to be a regular smoker had 7 ppm and higher exhaled CO. There was a statistically significant and positive correlation between exhaled CO levels and nicotine dependency after controlling for environmental tobacco smoke exposure and cigarettes per day (partial correlation, r=0.334, p=0.004). Exhaled CO can be used as a predictor of smoking status and environmental tobacco smoke exposure and an indicator of nicotine dependency in adolescents.