The clinical usefulness of laboratory testing of adult patients with venous thromboembolism (VTE) for heritable thrombophilia needs to be critically evaluated. At present, some clinicians use testing to identify patients at higher risk of recurrence (who may benefit from an extended period of anticoagulation beyond the usual 3-6 months) and their relatives at risk of a first VTE episode. As prevalence of heritable thrombophilia is related to age and ethnic origin, the pretest probability of detecting heritable thrombophilia may be low in unselected populations. Interpretation of laboratory results may not be straightforward. Apparent deficiencies of a natural anticoagulant may be due to acute thrombosis and "consumption", concomitant therapy with heparin and/or warfarin and other clinical factors. The predictive value of recurrent VTE conferred by the most common types of heritable thrombophilia (factor V Leiden and the G20210A prothrombin mutation) is limited. Risk of recurrence associated with deficiencies of a natural anticoagulant is less certain due to their rarity. Clinical risk factors (eg, the presence or otherwise of provoking factor(s) and whether or not the risk factor for VTE is reversible or permanent) appear to be the most important predictors of VTE recurrence. Duration of anticoagulation should be determined by clinical risk factors rather than the presence, or otherwise, of heritable thrombophilia. The benefit of identifying relatives who are carriers of thrombophilia is uncertain, as VTE is a multifactorial disease resulting from the interaction of various risk factors, some well recognised and others as yet unknown.