The average risk is just one event in 5,000 flights, but this can increase depending on the length of the flight, frequency of flying, whether you are a woman - and especially one taking the pill - and whether you are short or tall.
Researchers assessed the risk after surveying 8,755 international corporate employees who regularly flew long-haul, defined as any flight that is longer than four hours.
The risk of DVT or venous thrombosis was greatest in the eight weeks following a long-haul flight, and the absolute risk was one event per 4,656 flights.
Short passengers were at greater risk if their feet didn't touch the floor of the plane, as were tall passengers if they were sitting in a cramped position.
(Source: Public Library of Science Medicine (PLoS Med), 2007; 4: 1508-14).