Data from a cross-sectional random population sample of 10,359 middle-aged Scottish men and women are used to investigate the relationships between self-reported tobacco consumption and three biochemical markers of tobacco inhalation: expired air carbon monoxide (CO), serum thiocyanate and serum cotinine. These data represent one of the largest samples of these biochemical markers yet analysed. The results show that, for each sex, the biochemical markers are highly correlated for smokers and for the entire sample of mixed smokers and non-smokers. CO is the preferred biochemical marker, in such groups, because it is the cheapest, is non-invasive and gives virtually instantaneous results. Self-reported daily cigarette consumption also correlates well with each of these biochemical markers, and so it appears that people are, in the context of population studies, mainly truthful about their smoking. The relationships with self-reported cigarette consumption are curvilinear with apparent levelling out of the gradient at around 25 cigarettes/day for cotinine and thiocyanate and at greater than 40 cigarettes/day for CO. Sex differences are small, although thiocyanate is generally higher and cotinine generally lower in women with the same self-reported cigarette consumption as men. Amongst non-smokers, only cotinine is able to discriminate between self-reported levels of exposure to passive smoking. CO and thiocyanate are not suitable for measuring low levels of smoke inhalation, such as found in passive smokers.