Most users of synthetic-steroid hormones for postmenopausal hormone replacement or contraception, are healthy with a low background incidence of major diseases. Since they take these medicines for a long time, serious or frequent adverse effects should not be occurred. By changing content and reducing dose of both estrogen and progestagen, the incidences of venous and arterial thrombotic events by combined low dose oral contraceptives, is low in women who have not major risk factors. Although postmenopausal hormone replacement therapy decreases risk for coronary heart diseases, it seems to increases for venous thromboembolic events according to several clinical trials. In this article, the recent data on both the risk and the mechanisms of venous and arterial thrombotic events associated with use of oral contraceptives or with postmenopausal hormone replacement, is reviewed.