A point mutation in coagulation factor V which causes resistance to cleavage of factor Va by activated protein C (APC), was recently found to underlie thrombotic events. We examined 20 consecutive patients, under the age of 40, who suffered from idiopathic venous or arterial thrombosis. In 8 (40%) there was resistance to APC manifested by absence of the expected prolongation of activated partial thromboplastin time (aPTT). In 3, the addition of normal plasma corrected the anomaly in the patient's plasma, although the addition of factor V- deficient plasma caused no change. In a family of a 17-year-old boy with idiopathic deep venous thrombosis we found a mutation in factor V which was responsible for APC resistance. The patient and 4 family members showed a single G to A transition in position 1691 in their cDNA, resulting in substitution of arginine (506) for glutamine. The mutation in this area, which is the cleavage site for APC, is associated with thrombotic episodes and is frequently observed in patients with familial thrombophilia.