In preventive studies of company employees in Westphalia, HDL cholesterol was measured in the sera of 4933 men and 2365 women, as well as HDL apolipoprotein A-I in the sera of 3509 men and 1648 women. Three subgroups were compared: non-smokers = persons who have never smoked; ex-smokers = persons who do not smoke at present but did in the past; smokers = persons who smoke cigarettes at present. Mean values for HDL cholesterol and for HDL apolipoprotein A-I were significantly lower in smokers than in non-smokers or in ex-smokers, while there were no differences between the last two groups. These differences appeared in both sexes but were more pronounced in women than in men. To answer the question whether the observed differences are caused by the consumption of cigarettes by itself or whether they are caused by the presence of other risk factors, further subgroups were compared to assess the influence of the risk factors obesity, hypertension, hypercholesterolaemia, hyperglycaemia and hyperuricaemia. It was found that - regardless of the presence of no, one, two or more risk factors - the frequency of probands with low HDL cholesterol values (less than 0.907 mmol/l (men); less than 1.166 mmol/l (women] was about 10% higher in smokers than in non-smokers or ex-smokers. Subgroups based on the number of risk factors did not exhibit the same clear distribution for apolipoprotein A-I values as were seen for HDL cholesterol. The results are interpreted in the light of the existing literature.