Platelet function was investigated in 11 healthy non-smokers, 11 medium smokers (less than 10 cigarette/day) and 11 heavy smokers (greater than 20 cigarette daily). Cigarette consumption was in medium smokers 6.2 cigarette/day, and in heavy smokers 30.7 cigarette/day. Erythrocyte count, platelet count, plasma levels of cholesterol and triglycerides, and circulating platelet aggregates showed no significant difference between smokers and non-smokers. Thromboxane B2 level in plasma was higher in heavy smokers (20.1 +/- 17.3 pg/0.1 ml) than in non-smokers (58.8 +/- 37.1 pg/0.1 ml) and medium smokers (65.4 +/- 32.1 pg/0.1 ml) (P less than 0.01). Platelet factor 4 level was high in non smokers prior to smoking 1 cigarette and normal in medium and heavy smokers. Plasma concentration of beta thromboglobulin and 6-ketoprostaglandin F1 alpha showed no significant changes between non-smokers and heavy smokers. No dependence on smoking 1 cigarette was found in any group. The data suggest that long term cigarette smoking increases platelet activity manifested by an increased activity of thromboxane A2. Cigarette smoking contributes to the development of coronary artery disease also by means of activation of platelet functions.