Stools containing altered blood-plasma urea: creatinine ratio as a simple test for the source of bleeding. The plasma urea:creatinine ratio (U:C ratio) is known to be elevated in cases of upper gastrointestinal bleeding. Almost all patients with haematemesis have upper gastrointestinal (or generalized) bleeding so that in this study we characterized the diagnostic power of the U:C ratio in patients with stools containing altered blood without haematemesis in the hope that this simple laboratory test (used in conjunction, perhaps, with clinical data) might reduce the number of patients subjected to an unrewarding gastroscopy or colonoscopy. Of 76 cases seen in a provincial and a metropolitan hospital, 42 and 34 patients had upper and lower gastrointestinal bleeding, respectively. Fifty-four per cent of those with upper gastrointestinal bleeding and none of those with lower gastrointestinal bleeding had U:C ratios above 110 on admission. However, a discriminating level of 90 is considered to be more suitable, judged by the quadratic uncertainty score. At this level the odds for upper gastrointestinal bleeding were 15:1. 