Disorders of coagulation during ascent to high altitude in the unacclimatized are well recognized. In order to document the sequence of haematological changes, the bleeding time was estimated weekly in six climbers during a six week climb. The median bleeding time was shown to increase by 50% between the first and fourth week, it then plateaued during continued ascent to 5700 m. During the sixth week the bleeding time rapidly reverted to baseline levels whilst still at altitude. The observations might be explained by the compensatory changes in coagulation factors following the initial prothrombotic phase, compounded by increased capillary fragility. Acclimatization to altitude is accompanied by a series of complex haematological changes. The temporal sequence should be considered when devising a strategy to reduce the risk of thrombosis.