This study assesses serum thiocyanate and expired-air carbon monoxide based on findings for 8,895 participants in the Multiple Risk Factor Intervention Trial. For each marker and combination of markers, a logistic probability model was used to generate a plot of true positive vs false positive rates (an operating characteristic curve), an optimal cutpoint, and the positive predictive value for discriminating tobacco smokers (cigarettes, pipes, cigars, or cigarillos) from non-tobacco smokers. Optimal cutpoints were 83 μmol/liter for thiocyanate and 10 ppm for carbon monoxide. These cutpoints were applied to the entire group to evaluate accuracy. Specificity was higher for carbon monoxide than for thiocyanate (91.8 vs 87.3%) and sensitivity was lower (85.7% for carbon monoxide vs 88.8% for thiocyanate). When carbon monoxide and thiocyanate were both used, specificity was 92.7% and sensitivity was 89.1%. The estimated cutpoints derived from the Multiple Risk Factor Intervention Trial result in predictive values greater than 87% when the prevalence of smokers is greater than 50%. Cutpoints should be modified to maximize the predictive value when the prevalence of smokers is less than 50%. Both serum thiocyanate and carbon monoxide are good markers of smoking exposure. Serum thiocyanate may be preferable to carbon monoxide because of its longer half-life.