The predictive value of a urea/creatinine ratio > or = 100 for indicating upper gastrointestinal bleeding was evaluated in 78 consecutive patients with a history of gastrointestinal bleeding 24 hours or less before admission. Serum beta-2-microglobulin was measured to elucidate whether renal hypoperfusion plays a part in elevation of the urea/creatinine ratio. The predictive value of a urea/creatinine ratio > or = 100 in terms of upper gastrointestinal bleeding, was 95% (95% confidence interval (CI): 83-99%), whereas the predictive value of a urea/creatinine ratio < 100, indicating lower gastrointestinal bleeding, was 41% (CI: 25-59%). No statistically significant differences were found between serum beta-2-microglobulin levels in upper and lower gastrointestinal bleeders. We conclude that a urea/creatinine ratio > or = 100 strongly indicates an upper gastrointestinal bleeding source and that the main cause of a high ratio is not renal hypoperfusion.