The authors analyse the incidence of thromboembolic complications during combined oral contraception. Among genetic factors predisposing to venous thrombosis and thromboembolic disease, they present, with the help of three case histories, the molecular biology of activated protein-C resistance, the newly discovered and most frequently implicated pathogenetic factor of inherited thrombophilia. Among 350 asymptomatic women requesting prescription for an oral contraceptive they found a 9.1% frequency of heterozygote Leiden-mutation carriers. For the prevention of a rare, however, almost always life-threatening complication the authors recommend activated protein C-resistance analysis to be carried out whenever an oral contraceptive is prescribed for the first time. Positive results will provide life-time aid in avoiding factors which increase the risk of thromboembolic disease. Negative results may help reducing the number of those who, risking an unwanted pregnancy, are hesitant to start on oral contraceptives for their implication in thromboembolic disease.