The concurret use of smoking and oral contraceptives affects the hemostatic balance, thereby inducing a thrombophilic state. In order to clarify the effects of this association on the hemostatic system, the possible changes in the markers of activation of coagulation (thrombin-antithrombin III complexes and prothrombin fragment F1+2) were evaluated in 35 women given a third-generation oral contraceptive for 6 months; 13 of these women (37.1%) were mild or moderate smokers. No differences were found in basal levels of the coagulation and fibrinolytic parameters between smokers and non-smokers. During oral contraceptive administration, both F1+2 fragment and thrombin-antithrombin III complex concentrations significantly increased both in smokers and in non-smokers (p < 0.01). Fibrinogen plasma levels increased in both groups (p < 0.01). Antithrombin III activity was reduced in both groups during treatment, but the difference was significant only in smokers (p < 0.05). Although the sample size of smokers was too small to draw definitive conclusions, present results appeared to confirm previous data about the effect of the concurrent use of smoking and oral contraceptives on antithrombin III levels, but did not demonstrate any additional effect of moderate smoking on the activation of the clotting system induced by this oral contraceptive preparation.