OBJECTIVE To determine the frequency of and risk factors for myocardial infarction (MI) in patients admitted to an intensive-care unit (ICU) with gastrointestinal (GI) hemorrhage and to ascertain the effects on mortality and lengths of stay. METHODS Demographic, laboratory, and outcome data were determined for all patients admitted to a medical ICU with GI hemorrhage between April 1996 and January 1997. Serial creatine kinase with isoenzyme levels and electrocardiograms were interpreted blindly by a senior cardiologist. RESULTS For 83 consecutive admissions to the ICU because of GI hemorrhage, the patients' mean (+/- standard error) age was 65.0 +/- 1.7 years and APACHE II (acute physiology and chronic health evaluation) score was 15.7 +/- 0.8. In-hospital death occurred in 16 patients (19%). Patients who did not survive had a lower admission systolic blood pressure (99.2 +/- 4.5 versus 115.0 +/- 4.0 mm Hg; P = 0.01) than did those who survived. Eleven of 83 patients (13%) fulfilled both enzymatic and electrocardiographic criteria for MI. Ten patients (12%) had electrocardiographic evidence of myocardial ischemia but did not meet criteria for MI. Patients with MI were older (74.4 +/- 4.0 versus 61.7 +/- 2.0 years; P < 0.05), had a higher acuity of illness (APACHE II score, 21.6 +/- 3.0 versus 14.6 +/- 0.7; P < 0.05), and had more coronary risk factors (2.3 +/- 0.3 versus 1.4 +/- 0.1; P < 0.05) in comparison with those without MI or ischemia. Patients with MI also had longer ICU (8.6 +/- 2.4 versus 3.3 +/- 0.4 days; P < 0.05) and hospital (16.3 +/- 3.4 versus 9.1 +/- 0.8 days; P < 0.05) lengths of stay. Patients older than 65 years had a threefold increased risk (risk ratio, 4.0; 95% confidence interval, 0.9 to 17.4) and those with two or more risk factors for coronary artery disease had a ninefold increased risk of MI (risk ratio, 10.2; 95% confidence interval, 1.4 to 76.1) in comparison with those who were younger or who had fewer coronary risk factors, respectively. MI complicating GI hemorrhage did not significantly affect the risk of in-hospital mortality (risk ratio, 1.5; 95% confidence interval, 0.5 to 4.4). CONCLUSIONS MI occurs frequently in patients with GI hemorrhage admitted to an ICU. Age more than 65 years and two or more risk factors for coronary artery disease identify patients who are at greatest risk for occurrence of MI, which is associated with longer ICU and hospital stays.