The aim of this study was to investigate the levels of ExpCO and its effect on giving decision to stop smoking in primary care. The study was held in Tekkeköy Family Health Center. A total of 853 current smokers (391 in study group and 462 in control group) included into the study. Both group of smokers got first two steps of “5 A” method (Ask, Advice, Assist, Assess and Arrange) for smoking cessation however the ExpCO levels were measured only in the study group. CO levels in expiratory air were measured by the single breath method using a calibrated carbon monoxide monitor and the time of the last smoked cigarette (TLC) of the participants were examined. The mean ExpCO level was 16.9±7.7 ppm in study group. The mean Fagerstrom Nicotine Dependency Test (FNDT) score was 5.0±2.7 in both groups. The mean package/ year value in the both participants were 16.0±14.8 (min=1, max=100) and the mean Fagerstrom Nicotine Dependence Test (FNDT) score was 5.0±2.7. The mean ExpCO level was 15.7±8.7 ppm in study group. A significant positive correlation was determined between FTND score and ExpCO. TLC values in males were significantly lower than females (p<0.001) in both groups. In a linear regression model it was seen that age, package/year value, FNTD scores and TLC are independent risk factors for elevation of ExpCO values. 23 (5%) smokers in the control group and 47 (12%) in the study group decided to quit smoking (x2=15.412, p<0.001). This measurement might have an effect on motivating smokers to quit smoking.