Thirty-six young, healthy, nonsmoking women have been selected to check the effect of low-dose oral contraceptives on hemostasis. Two identical groups were treated by Marvelon (a monophasic oral contraceptive containing ethinyl estradiol and desogestrel) or Trigynon (a triphasic oral contraceptive containing ethinyl estradiol and levonorgestrel) for a 6-month period. In the absence, previously controlled, of substantial differences between the effects of each treatment on hemostasis, all the results were pooled at the third and sixth month of the study. The effects of oral contraceptive treatment were as follows: (1) platelet number, platelet aggregating ratio, and plasma beta-thromboglobulin level were not significantly altered, and (2) antithrombin III activity was not reduced despite a slight decrease or antigen concentration. The von Willebrand factor parameters, factor VIII:C, factor VII:C, and clottable fibrinogen were significantly increased. Plasminogen (activity and antigen concentrates) and alpha 2-antiplasmin levels were also significantly increased. Activated partial thromboplastin time and euglobulin lysis time measured after venous occlusion were significantly shortened. Although statistical analysis did not show dramatic changes in all these parameters, some individual extreme values were substantially altered. Therefore we believe that these later values are worthy of cautious consideration for weighing the role that hemostasis factors might play in individual thrombotic risk.