Objective: To measure exhaled carbon monoxide (COex) levels in smokers with and without COPD. Methods: Smokers treated at outpatient clinics of São Lucas Hospital in the city of Porto Alegre, Brazil, between September of 2007 and March of 2009 were invited to participate in this study. The participants completed a questionnaire regarding demographic and epidemiologic characteristics and were submitted to spirometry, as well as to determination of COex and urinary cotinine levels. The participants were divided into two groups: those with COPD and those without COPD. Results: The study involved 294 smokers, of whom 174 (59.18%) had been diagnosed with COPD. All of the participants presented with urinary cotinine levels > 50 ng/mL. Smokers with COPD presented significantly higher median values for age and pack-years than did those without COPD (p < 0.001 and p = 0.026, respectively). No other statistically significant differences were found. When adjusted for gender, age at smoking onset, number of cigarettes/day and urinary cotinine level, the mean values of COex were higher, but not statistically so, in the COPD group than in the non-COPD group (17.8 ± 0.6 ppm and 16.6 ± 0.7 ppm, respectively; p = 0.200). The differences remained nonsignificant when plotted logarithmically. A wide dispersion of COex values was found when the participants were classified by FEV1 level (r = −0.06; p = 0.53) or by Global Initiative for Chronic Obstructive Lung Disease classification (r = 0.08; p = 0.34). The proportions of false-negative results for smoking were 18.4% and 6.7%, respectively, in the COPD and non-COPD groups (p = 0.007). Conclusions: Since COex values did not differ significantly between smokers with COPD and those without, there seem to be no major contraindications to their use in smokers with COPD.

Keywords: Carbon monoxide; Smoking cessation; Pulmonary disease, chronic obstructive.