['To analyse socioeconomic differences in serum thiocyanate concentrations among current smokers, and whether such differences persist after adjustment for the number of cigarettes smoked.', 'General population of six districts of the Czech Republic in 1992.', '451 male and 282 female current smokers.', 'Serum concentration of thiocyanate.', 'There was a clear educational gradient in serum thiocyanate among male smokers; car ownership and crowding were not related to thiocyanate. Age adjusted mean concentrations in men with primary, vocational, secondary, and university education were 168.6, 158.2, 148.2, and 141.8 micromol/l, respectively (p for trend 0.032). Adjustment for the average daily number of cigarettes explained a part of this gradient. Socioeconomic differences in serum thiocyanate were not seen in women.'] Is smoke intake among smokers higher in lower socioeconomic groups?