Severe thrombotic events following ovarian stimulation for in-vitro fertilization (IVF) procedures in three women are reported. None of these patients presented any concomitant clinical sign of ovarian hyperstimulation syndrome. Coagulation inhibitors were in the normal range but cardiovascular risk factors were present. It is postulated that early thrombosis could be favoured by high endogenous plasma oestrogen concentrations subsequent to ovarian stimulation when associated with another risk factor. Our data are discussed in relation to previous publications. It is suggested that risk factors must be considered individually before each IVF attempt. In patients at high risk, clinical management of the post-transfer period is recommended.