The relationship between smoking habits and plasma thiocyanate levels has been evaluated in four adult samples of the general population containing men and women aged 20-84 and in a sample of 11-year-old non-smoking children of both sexes, for a total of 7577 individuals. Mean levels of plasma thiocyanate was found be approximately 20 mumol/l in children, 30 mumol/l in non-smoking adults and increasingly higher in smoking adults. The slopes of the regression equations of thiocyanate on cigarettes smoked per day range from 3.041 to 5.740, with correlation coefficients of from 0.638 to 0.809. In another occupational sample of 2802 men aged 46 to 65, where the correlation coefficient (between cigarette consumption and thiocyanate) was only 0.49, plasma thiocyanate was a better predictor of 4 year fatal events than cigarette consumption, 4 other covariates being considered in the same multivariate model.