59 cases of overt gastrointestinal haemorrhage were reviewed in which the source of bleeding was not initially evident but was identified by later investigation. The admission plasma urea to creatinine concentration ratio proved highly accurate in distinguishing upper and lower gastrointestinal sources of haemorrhage. Regardless of blood volume lost, 33 of 38 patients with upper gastrointestinal haemorrhage (87%) had a ratio of 100 or more, and 20 of 21 patients with lower gastrointestinal haemorrhage (95%) had a ratio of less than 100, an overall accuracy of 90%. Elevation of the ratio was not related to clinical evidence of hypovolaemia. This simple ratio may be valuable in determining the sequence of investigations in patients with an unidentified source of gastrointestinal haemorrhage.