Exposure to hypobaric hypoxia demands maximum effort of the body and can lead to high altitude illnesses. Recently, there is rising interest on coagulation activation during trekking and mountaineering in higher regions and on development of oxidative stress due to hypoxia. 30 volunteers have been examined during an high altitude research expedition to the 7134m high mount Pik Lenin in Kyrgyzstan to investigate mechanisms of coagulation activation and effects of antioxidant supplements on oxidative stress.