The term thrombophilia describes an increased tendency to clinical thrombosis associated with laboratory evidence of abnormal haemostasis. Causes include inherited deficiencies of natural anticoagulants (antithrombin, protein C, and protein S), polymorphisms causing resistance to activated protein C (factor V Leiden mutation) or disturbing the normal proclot or anticlot balance (prothrombin G20210A mutation), and disorders which are polygenic or interact with dietary and environmental factors (high factor VIII and hyperhomocysteinaemia). Inherited thrombophilias, most commonly factor V Leiden and high VIII, are risk factors in most cases of venous thromboembolism under the age of 40.1 It is therefore tempting to assume that a similar relation will be found in arterial thrombosis, especially as stroke in young people often remains unexplained even after extensive investigation of other possible causes. In the previous issue of thisJournal, pp 508−511 …