An optimal cutoff point of expired-air carbon monoxide (Ex-CO) for detecting smokers should be determined in terms of its sensitivity and specificity and the prevalence of smoking in the target population. The purpose of this study is to determine the optimal cutoff point of Ex-CO for detecting smoking males in a Japanese community whose smoking prevalence was over 50%. Among free-living residents in a rural population, "true smokers" determined by presence of cotinine in serum were 61 % (n=94). When Ex-CO at 7 ppm or over differentiated "smokers" from "non-smokers", sensitivity and specificity for detecting smokers was 0.93 and 0.95, respectively, which comprised the best Youden's index. This setting also produced the minimum percentage of misclassified cases. In conclusion, 7 ppm of Ex-CO, which is exceptionally low value relative to the western standard, appears to be the most optimal cutoff point for a survey in a population with such high smoking prevalence.