A previously unrecognized mechanism for thrombophilia, characterized by a poor anticoagulant response to activated protein C (APC), was recently described by Dahlback et al (1993) in several families with an inherited tendency to thrombosis. This poor response to APC was then shown to be due in the large majority of cases, to a selective defect in factor V (R506Q or Factor V Leiden mutation), a factor that not only expresses procoagulant properties but also plays a role in the anticoagulant system as an important target of APC (Bertina et al, 1994; Dahlback and Hildebrand, 1994). The so-called APC resistance has been found to be the most frequent alteration in previously undiagnosed thrombotic patients with prevalence ranging from 20% to 65% in different studies (Faioni et al, 1993; Griffin et al, 1993; Koster et al, 1993; Cadroy et al, 1994; Cusman et al; 1994; Halbmayer et al, 1994; Legnani et al, 1994; Sevenson and Dahlback, 1994). On the Basis of these results APC resistance seems far from being a rarity, it has therefore been suggested that all thrombotic patients should be tested for this abnormality (Koster et al, 1993).