There has been a documented shift towards increasing age and severity of illness in the patient population undergoing cardiac surgery. To determine if there was a coincident change in frequency, gastrointestinal (GI) complications were prospectively recorded in a consecutive series of 5,438 patients undergoing cardiac surgery from 1983 to 1991. There were 73 complications in 69 patients (incidence = 1.4%) defined as any GI condition that required transfer to an acute care unit, surgical intervention, blood transfusion, or treatment that prolonged the hospital course. Fourteen patients died, a mortality rate of 20% for patients with GI complications (p < 0.001 versus patients without GI complications). The most frequent complications were those of gastric ulceration despite routine use of H 2-receptor blocking agents. Thirty-six patients had upper GI (UGI) bleeding from gastric ulceration with 4 patients requiring operative intervention to control hemorrhage and 6 fatalities in patients with UGI bleeding. Two additional patients died of septic complications following gastroduodenal perforation or penetration. Six patients experienced bowel obstruction or prolonged bowel dysfunction (three Ogilvie's sydrome) with two requiring laparotomy. There were four cases of cholecystitis, two cases of pancreatitis, and the remaining cases were equally divided among common septic complications (diverticulitis and ischemic injury among others). Three patients with massive intestinal infarction died. GI complications were significantly associated with older patients (p < 0.01) and valve surgery (p = 0.002) but were not more common in women. When considered as a separate group, patients with acid-peptic complications had longer perfusion times, increased use of vasopressors, and more frequent utilization of the intra-aortic balloon pump. In contrast to prior studies, this investigation indicates that GI complications associated with acidpeptic erosion of the UGI tract tend to occur in a different patient group than those with other GI complications. Older patients and/or those with a prior history of peptic ulcer symptoms, as well as those who experience prolonged perfusion times, low cardiac output, or prolonged ventilatory support, should be under rigorous gastric pH surveillance and receive aggressive prophylactic treatment with high-dose Ha antagonists, antacids, and/or sucralfate.