The aetiology of portal vein thrombosis (PVT) in adults is complex. Risk factors include local precipitating factors and acquired and inherited factors, an area in which there has been much recent progress. Although PVT in the absence of cirrhosis may be regarded as a somewhat different disorder to PVT in the presence of cirrhosis, in both cases most studies support a role of the prothrombin G20210A mutation. Some differences in risk factors observed between different studies may relate partly to referral patterns or study design, although individual patients may develop PVT as a result of differing combinations of risk factors. The demonstration of an inherited thrombophilic mutation in a subset of PVT may ultimately inform clinical management regarding the use and duration of anticoagulation therapy, although there is a need for evidence from randomized-controlled clinical trial data.