BACKGROUND Pulmonary emboli (PE) can have potentially fatal consequences. Inferior vena caval filters (VCFs) are metal alloy devices that mechanically trap fragmented thromboemboli from the deep leg veins en route to the pulmonary circulation. Filters are designed to be introduced (and in the case of retrievable filters, removed) percutaneously. Although their deployment seems of theoretical benefit, their clinical efficacy and adverse event profile is unclear.This is an update of a Cochrane review first published in 2007. OBJECTIVE To examine evidence for the effectiveness of VCFs in preventing pulmonary embolism (PE). Secondary outcomes were mortality, distal (to filter) thrombosis, and filter-related complications. METHODS The Cochrane Peripheral Vascular Diseases Group searched their Specialised Register (last searched October 2009) and the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library 2009, Issue 4 for randomised or controlled clinical trials of VCFs for the prevention of PE. The authors contacted filter manufacturers for information. METHODS Controlled clinical trials (CCTs) and randomised controlled trials (RCTs) that examined the efficacy of filters in preventing PE. METHODS Two authors independently extracted information. RESULTS Two studies were included involving a total of 529 people. One open quasi-randomised trial of 129 participants with traumatic hip fractures showed a reduction in PE but not mortality over a 34 day period in the filter group. The PREPIC (Prévention du Risque d'Embolie Pulmonaire par Interruption Cave) trial, was an open RCT of 400 participants with documented proximal deep vein thrombosis (DVT) or PE who received concurrent anticoagulation. Permanent VCFs prevented PE at eight years. No reduction in mortality was seen, but this reflected an older study population; the majority of deaths were due to cancer or cardiovascular causes. There was an increased incidence of (DVT) in the filter group. Adverse events were not reported. CONCLUSIONS No recommendations can be drawn from the two studies. One study showed a reduction in PE rates but not mortality, but was subject to significant biases. The PREPIC study lacked statistical power to detect a reduction in PE over shorter and more clinically significant time periods. However, the trial demonstrated that permanent VCFs were associated with an increased risk of long term lower limb DVT.There is a paucity of VCFs outcome evidence when used within currently approved indications and a lack of trials on retrievable filters. Further trials are needed to assess vena caval filter safety and effectiveness.