In a number of women the use of combined oral contraceptives (OC) may induce the production of anti-ethinylestradiol antibodies circulating as immune complexes (IC) and which precipitate when the serum is brought at 25% saturation in ammonium sulphate. Testing for tests ICs in 50 women who had never taken OCs, in 100 women on OCs but otherwise healthy, and in 17 women on OCs presenting with vascular thrombosis showed that 20% of all healthy women and more than 95% of women with thrombosis were immunoreactive. The statistically significant correlation found between thrombosis and the presence in the serum of ICs and anti-ethinylestradiol antibodies suggests that an immune mechanism might be responsible for the vascular lesions induced by oral contraceptives containing synthetic estrogens.