Recently, the Swiss Medical Association and the Swiss Federal Office for Public Health started a campaign against smoking. Portable carbon monoxide (CO) devices measuring alveolar CO concentration are now available. It is important to know how accurate they are in discriminating smokers (S) from nonsmokers (NS) in different environmental conditions. We administered a smoking questionnaire and measured alveolar CO concentration in 241 people living in an urban area (PU) and 112 people living in a mountain area (PM). Smoking prevalence was 39% in PU and 17% in PM. Alveolar CO concentration was significantly higher in S than in NS (27 +/- 11, 11 to 60, vs. 14 +/- 9, 4 to 52, p < 0.01 in PU and 23 +/- 13, 4 to 48, vs. 9 +/- 7, 4 to 46, p < 0.01 in PM). We found significantly higher CO-values in NF of PU than PM (p < 0.01). This test has a sensitivity of 80%, a specificity of 78%/79% (PU/PM) and a predictive positive value of 74%/80% (PU/PM). The cut-off point has to be adapted depending on the environmental exposure (PU: 15 ppm, PM: 10 ppm CO). Alveolar CO measurement is a cheap accurate method whose results are immediately available to the general practitioner in his fight against smoking. This method merits higher application.