text: As mentioned by the first answerer (is that even a word) the mayor risk is PE (pulmonary embolism). PE happens when a trombous (aka blod clot) forms in your legs and then migrate to your lungs. \n\nDVP (deep venous trombosis) is associated with flights over 3000 miles. Also, the risk of getting it increases if you smoke, have varicose veins in your legs, you're a woman on the Pill, have a clotting disease (like Leiden's V factor, VonWillebrand's disease...)\n\nWhile you're on a plane, get up at least once every hour, move your leg muscles while sitting, and you should be OK. If you have any disease (not just the ones I mentioned above), go see your doctor about  DVT profilaxis, a medication used to prevent the clots from forming.