In a Norwegian health study involving 25,300 persons the mean serum thiocyanate level in non-smokers was 33.9 mumol/l for males and 33.5 mumol/l for females. In moderate smokers (five to nine cigarettes per day) the mean level was 59.6 mumol/l for males and 70.9 mumol/l for females. In heavy smokers (greater than 25 cigarettes per day) the mean level was 87.3 mumol/l in males and 99.7 mumol/l in females. The difference between the thiocyanate levels in females and males smoking the same number of cigarettes can be explained by the sex difference in distribution volume for thiocyanate. Among non-smokers the mean level of serum thiocyanate was the same whether the persons had been indirectly exposed to tobacco smoke or not. The mean serum thiocyanate levels were up to 10 mumol/l higher in the last half part of the year than in the first part. This can be explained by seasonal variations in the content of thiocyanate in the diet. The range of the individual thiocyanate level was great both in non-smokers and in smokers, resulting in a large overlap. Serum thiocyanate can therefore not distinguish all non-smokers from all smokers. However, by choosing suitable 'cut-off levels' it is possible to extract from the total population groups consisting of a large majority of smokers and only a few non-smokers and vice versa.