It is commonly accepted that women on oral contraceptive therapy have about a four fold increased incidence of venous thrombosis in comparison to non users. Women with FV Leiden polymorphism have an even higher incidence. The purpose of the paper is to show that women with FV Leiden polymorphism may sometimes remain asymptomatic in spite of long-term use of oral contraceptives. We have studied and followed 37 women with this polymorphism (35 heterozygotes and 2 homozygotes) who remained asymptomatic even after a long use, occasionally up to 10-12 years of oral contraception. Furthermore, these women remained asymptomatic in spite of the fact that the majority of them took preparations containing third generation progestins (gestodene or desogestrel). These progestins are considered to be more thrombogenic as compared to older ones. Finally, several of these women became pregnant before, during interruptions or after the contraceptive therapy and remained also asymptomatic but for one patient with varicose veins who developed superficial phlebitis during one pregnancy. These data indicate that FV Leiden polymorphism, as far as oral contraceptive therapy is concerned, is not a very strong prothrombotic condition and probably does not represent an absolute contraindication to its use. Unfortunately so far there is no sure way to distinguish the women with this polymorphism who will develop venous thrombosis from those who will remain asymptomatic during oral contraceptive therapy.