BACKGROUND:The magnitude of the risk of venous thromboembolism (VTE) following air travel has been difficult to resolve due to a lack of adequate data. We determine the association more precisely by using a large dataset and an improved method of analysis. METHOD:Data on air-travel history for each of 5,196 patients hospitalised for VTE in Western Australia from 1981 to 1999 is analysed using a log-linear regression model for the probability that a flight triggers VTE and for the baseline hazard rate for VTE hospitalisation. RESULTS:The risk of VTE being triggered on the day of an international flight relative to a flight-free day is 29.8 (95% CI 22.4-37.3). Evidence that this relative risk depends on age is weak (p = 0.06), but the absolute risk clearly depends on age. The annual relative risk for an individual taking one international flight, compared with an individual of the same age taking no flight, is estimated to be 1.079. The estimated median time from flight to hospital admission is 4.7 days (95% CI 3.8-5.6) and the estimated 95th percentile is 13.3 (95% CI 10.3-16.8). CONCLUSIONS:Evidence for an association between international air travel and VTE hospitalisation is strong and passengers should be advised on ways to minimise risk during long flights. While 29.8 is a large relative risk, it must be remembered that the baseline risk is very small and the relative risk applies only to the unobserved triggering of a deep vein thrombosis episode on the day of travel; the consequent hospitalisation occurs on one of numerous ensuing days.