The objectives of the Wright (WHO Research Into Global Hazards of Travel) project are to confirm that the risk of venous thromboembolism (VTE) is increased by air travel and to determine the magnitude of risk, the effect of other factors on the risk and to study the effect of preventive measures on risk. This publication sets out the findings of epidemiological studies carried out during the first phase of the project, which indicate that the risk of VTE approximately doubles after a long-haul flight (more than 4 hours) and also with other forms of travel where travellers are exposed to prolonged seated immobility.