BACKGOUND/AIMS: Gastrointestinal (GI) bleeding remains a common medical problem, with morbidity and mortality rates of GI bleeding in intensive care unit (ICU) believed to have remained unchanged. There has been no report about the etiology and clinical manifestation of GI bleeding in ICU in Korea. Therefore, we performed this study to determine the frequency, etiology, risk factors, and outcome of clinically significant GI bleeding that occurred after admission to ICU. METHODS: We reviewed medical records of 1829 patients admitted to medical ICU in Samsung Medical Center from October 1994 to May 1999. Cases were enrolled the patients who developed GI bleeding more than 24 hours after admission to the medical ICU. The cases were compared with control populations: a set of ICU patients without GI bleeding matched with cases for age, gender, and length of ICU stay to evaluate the risk factors of GI bleeding. RESULTS: Clinically significant GI bleeding, confirmed by endoscopy, occurred in 71 patients of 1,829 patients (3.9%) after a mean ICU stay of 14+/-2.6 days. Gastric ulcer bleeding was the most common source of GI bleeding, accounting for 29.6% of cases overall. There were no statistical differences in underlying disease, mechanical ventilator use, heparin or steroid use, prothrombin time, prophylactic drug use such as H2 blocker and antacid use between cases and controls. However, thrombocytopenia (<50,000/mm3) was more common in cases who had GI bleeding than controls (P<0.05). CONCLUSIONS: Gastric ulcer was the most common cause of GI bleeding in ICU and careful attention was necessary to patients with thrombocytopenia (<50,000/mm3) in ICU.