Conditions which result in hypercoagulable blood or venous stasis may predispose to the development of deep vein thrombosis (DVT). Most of the recently described risk factors for DVT induce a hypercoagulable state. Over a 3-year period we have observed anomaly of the inferior vena cava (IVC) in four young patients presenting with spontaneous unprovoked DVT. This is a greater than expected rate (5% observed versus 0.5% expected). Further, bilateral DVT, which constitutes less than 10% of cases in most series, was present in three of the four cases. Anomaly of the IVC is a rare example of a prevalent congenital condition that predisposes to DVT, presumably by favouring venous stasis. This diagnosis should be considered in young patients with spontaneous and bilateral DVT.