Oral contraceptives increase the natural incidence of venous thrombosis of 1-2/10,000 women per year 3- to 4-fold. Recent studies have shown that desogestrel or gestodene containing formulations bear twice the risk of older low-dose ovulation inhibitors. During pregnancy, the incidence of thrombosis rises to 10/10,000 women-years and post partum up to 40/ 10,000. For 60% of thromboses no causal explanation can be found. In approximately 40% of the patients an inherited thrombophilia can be presumed. Among the hereditary types of thrombophilia, a resistance to activated protein C (APC-resistance) represents nearly 50%, while in 15 to 20% a deficiency of antithrombin III, protein C or protein S is found. APC-resistance, with a prevalence of 3-5% in the general population, increases the risk of thrombosis 8-fold and in users of oral contraceptives 35-fold. Antithrombin III-deficiency carries a comparable risk. Protein C-deficiency increases the risk of thrombosis 9-fold and in users of oral contraceptives 15-fold. Ovulation inhibitors do not influence the risk of thrombosis in women with protein S-deficiency. Anti-phospholipid-antibodies increase during treatment with oral contraceptives and represent a considerably enhanced risk of thrombosis. Inherent thrombophilia is suspected in a patient with a positive history or family history of thrombosis, especially with thrombosis before the age of 40 or with atypical localisation. Even in these risk groups, the cost-benefit ratio of selective screening is unfavorable, as today at most 70% of the hereditary thrombophilias can be diagnosed by laboratory analysis, and only very few of the patients will actually experience a thrombotic event: only 3 of 1000 carriers of APC-resistance will suffer from thrombosis during oral contraception. On the other hand, a negative result of laboratory tests does not exclude a hereditary thrombophilic disorder. At present, it is unclear whether a selective screening process is superior to a careful assessment of individual and family history. A general screening, however, cannot be justified because of the unfavorable cost/benefit ratio. If the individual or family history or pathological laboratory parameters indicate an increased risk of thrombosis, this risk has to be carefully weighed against the consequences of discontinuation of pill use. Those few individuals with risk factors who will experience a thrombo-embolic event, cannot be identified in advance. If oral contraceptives represent a particularly high risk in patients with thrombophilic disorders and/or other risk factors, other contraceptive methods should be considered. If a patient with risk factors decides on the use of oral contraceptives, she must be informed that in the case of symptoms indicating a thrombosis, a physician should be consulted immediately. The earlier an appropriate therapy is initiated, the more effectively pulmonary thrombo-embolism and permanent damage, such as the post-phlebitic syndrome, can be prevented.