The association of cigarette smoking with the development of occlusive vascular disease is firmly established. Unfavourable changes in a series of variables held independent risk factors for the development of vascular lesions (HDL-cholesterol, haematocrit, white blood cell count, fibrinogen and plasminogen activator inhibitor-1 (PAI-1)) are thought to be directly influenced by cigarette smoking. However, the role played by the genotype in the effect of smoking on the above parameters has not been investigated. To control the genotype, we studied the relationship between cigarette smoking and a series of cardiovascular risk factors in 27 monozygotic twin pairs (7 male and 20 female pairs, mean age +/- SD: 47.4 +/- 12.9 yrs) with a life-long discordance for smoking. Smoking twins had a life-long dose of smoking (Brickman index) of 287.3 +/- 241.5. Body mass index, blood pressure, haematocrit, haemoglobin and red blood cell counts, total cholesterol levels and the acute phase reactants alpha 1-acid glycoprotein and C-reactive protein were similar in smokers and non-smokers. Triglyceride was higher by 12.6% (9.5-35%, 95% confidence interval, p = 0.02) and HDL-cholesterol lower by 7.5% (0.2-15%, p = 0.04) in the smoking co-twins, who also had 8.4% (-0.2-17%, p = 0.06) higher white blood cell counts and 4.1% (1.2-7%, p < 0.01) larger mean platelet volume. There was no significant difference in clottable fibrinogen (by two methods) or in the activity of plasminogen activator inhibitor-1 between the two groups, nor was the within-pair difference in these parameters related to the smoking dose. Echo-doppler examination of the carotid arteries of 24 twin pairs showed mostly minor atherosclerotic lesions in 46% and 42% of the smoking and non-smoking co-twins. After adjustment for age, systolic blood pressure and platelet count and volume were the only variables significantly associated to the presence of vascular lesions. Cigarette smoking is associated with an atherogenic lipid profile and with changes in platelets and white cells potentially reflecting endothelial cell damage. When controlling the genotype, fibrinogen and PAI-1 activity levels did not seem directly influenced by cigarette smoking.