Abnormal coagulation factor V may underlie the thrombotic events associated with resistance to activated protein C (APC). We analysed 27 consecutive patients with documented idiopathic (recurrent) thromboembolism for the occurrence of point mutations within the APC sensitive regions of blood coagulation factor V. In 10 patients we observed a single basepair mutation resulting in a substitution of Arg506 to Gln. This mutation was significantly linked to in-vitro resistance to APC in these subjects. This mutation at Arg506 of factor V may form the molecular basis for the thrombotic events associated with APC resistance.