The mortality rate after surgery for upper gastrointestinal bleeding is high, and cannot be explained by the blood loss as such, or by the operation itself. Our immune defence is suppressed by haemorrhagic shock, blood transfusion and surgery. The effect is cumulative, and in many of these patients the cause of death is a consequence of severe immune suppression. The suppressed immune state is brought about by mediators that are produced, released or impaired by various trauma. As of today, we cannot regulate our immune reactions in acute situations in an effective way. Accordingly, we must try to prevent the development of severe immune suppression. The first step in reducing the high mortality rate in patients with acute upper gastrointestinal bleeding is to identify, as soon as possible, persons who risk experiencing a second bleeding after the initial episode. Our present knowledge allows such identification with a rate of accuracy of approximately 70%-80%.