Purpose: To describe the prevalence of, risk factors for, and prognostic importance of gastrointestinal (GI) bleeding and use of acid suppressants in acutely ill adult intensive care patients. Methods: We included adults without GI bleeding who were acutely admitted to the intensive care unit (ICU) during a 7-day period. The primary outcome was clinically important GI bleeding in ICU, and the analyses included estimations of baseline risk factors and potential associations with 90-day mortality. Results: A total of 1,034 patients in 97 ICUs in 11 countries were included. Clinically important GI bleeding occurred in 2.6 % (95 % confidence interval 1.6–3.6 %) of patients. The following variables at ICU admission were independently associated with clinically important GI bleeding: three or more co-existing diseases (odds ratio 8.9, 2.7–28.8), co-existing liver disease (7.6, 3.3–17.6), use of renal replacement therapy (6.9, 2.7–17.5), co-existing coagulopathy (5.2, 2.3–11.8), acute coagulopathy (4.2, 1.7–10.2), use of acid suppressants (3.6, 1.3–10.2) and higher organ failure score (1.4, 1.2–1.5). In ICU, 73 % (71–76 %) of patients received acid suppressants; most received proton pump inhibitors. In patients with clinically important GI bleeding, crude and adjusted odds for mortality were 3.7 (1.7–8.0) and 1.7 (0.7–4.3), respectively. Conclusions: In ICU patients clinically important GI bleeding is rare, and acid suppressants are frequently used. Co-existing diseases, liver failure, coagulopathy and organ failures are the main risk factors for GI bleeding. Clinically important GI bleeding was not associated with increased adjusted 90-day mortality, which largely can be explained by severity of comorbidity, other organ failures and age.