Serum high-density lipoprotein (HDL) cholesterol levels were assessed in a random sample of 939 men and 853 women ages 14-65 years in eastern Finland. Amount of daily smoking measured by serum thiocyanate concentration was negatively and independently associated with serum HDL cholesterol level and HDL/total cholesterol ratio in men. This negative association seemed largely to be due to low HDL cholesterol levels in heavy smokers. Both male and female current smokers had lower HDL cholesterol levels and HDL/total cholesterol ratios than those who had never smoked. This difference was found even after adjustment for age, body mass index, and beer drinking. The reduction of HDL cholesterol level seemed to be reversible, because those who had recently stopped smoking had higher HDL cholesterol levels and higher HDL/total cholesterol ratios than current smokers.