Exposure to the range of combustion products from wildland fires has been demonstrated to cause respiratory irritation and decreased lung function among firefighters. The measurement of carbon monoxide (CO) has been previously shown to be highly correlated with the range of contaminants found in wildland fires. In this article, we assess the feasibility of using a simple, noninvasive biological test to assess exposure to CO for a group of wildland firefighters. Measurements of CO exposure were collected using personal monitors as well as in exhaled breath for wildland firefighters who conducted prescribed burns in February-March 2004. Overall, the CO concentrations measured in this study group were low with a shift mean of 1.87 ppm. Correspondingly, the cross-shift difference in carboxyhemoglobin as estimated from exhaled breath CO levels was also low (median increase Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin)+0.2% carboxyhemoglobin). The use of exhaled breath measurements for CO has limitations in characterizing exposures within this worker population.