Can the blood urea nitrogen/creatinine ratio distinguish upper from lower gastrointestinal bleeding? We wanted to know if the blood urea nitrogen to creatinine (BUN/Cr) ratio could help distinguish upper from lower gastrointestinal bleeding. We analyzed retrospectively patients admitted to our hospital for gastrointestinal bleeding over the past 5 years. A total of 126 patients represented 74 upper bleeds and 52 lower bleeds. The mean BUN/Cr ratio was significantly higher in upper than lower bleeders, 34.8 and 17.8 respectively (p less than 0.001). No lower bleeder had a ratio of greater than or equal to 36, whereas 38% of upper bleeders had a ratio of greater than or equal to 36. The BUN/Cr ratio may be an easy, cheap method of distinguishing upper from lower gastrointestinal bleeding in some cases. A BUN/Cr ratio of greater than or equal to 36 suggests upper gastrointestinal bleeding, whereas a ratio of less than 36 is not helpful in locating the source of bleeding. 