Acute mesenteric ischemia is a rare but severe complication after open heart surgery. Its incidence (0.2-0.4%) is quite low, but mortality rates are ranging between 70% and 100%. From October 1992 to December 1996, 4,640 patients underwent open heart surgery with cardiopulmonary bypass: 74.6% coronary artery bypass graft (CABG) operations, 23.2% valve replacement including aortic repairs, and 2.2% corrections of congenital heart diseases or tumors of the heart. The overall mortality rate (30 days) was 3.4%, and after CABG, 2.9%. Twelve patients (0.26%), following CABG (one combined with aortic valve replacement, one with mitral reconstruction, and one with carotid disobliteration) developed signs of acute mesenteric ischemia in the early postoperative period (day 1 to 5). In all patients various abdominal symptoms, leukocytosis, acidosis, hyperlactatemia, hyperosmolality, renal failure, and, finally, hemodynamic instability were observed. Eleven patients underwent emergency laparotomy. Mesenteric angiography was done if possible in still stable patients (n=7); it showed severe stenosis or occlusion prior to the operation in each case. Other diagnostic methods were not reliable. In six patients (55%) during the first look, extensive bowel necrosis was found and in five patients an ischemic intestine but no necrosis was detected. Of these, three patients were affected by extensive bowel gangrene at the second look. In the fourth patient a disseminated peripheral ischemia of the entire small intestine was found intraoperatively. After mechanical release and stimulation normal bowel function could be reestablished. One patient underwent percutaneous transluminal angioplasty prior to the laparotomy. Bowel perfusion was still deteriorated but no necrosis was found intraoperatively. These patients were the only survivors in the investigated group; 10 of 12 patients (83.3%) died in the early postoperative period (day 1 to day 6). Predisposing factors for mesenteric ischemia are: arteriosclerotic patients after CABG (100%), age >70 years (91.7%), hyperosmotic dehydration (100%), and cardiac ischemia in 25%. Mesenteric ischemia is a fatal complication with high mortality rates after open heart surgery, especially in older, dehydrated patients with generalized atherosclerotic vessel disease. As the acute mesenteric ischemia usually starts during anesthesia or in the early postoperative period, setting of immediate diagnosis is very difficult. With the occurrence of typical symptoms diagnostic and therapeutic procedures (angiography and laparotomy) must be done very urgently owing to the life-threatening mesenteric process. When mesenteric gangrene already has taken place, the prognosis is very poor, despite extensive resection. Prevention can be exercised by avoiding perioperative hyperosmotic dehydration of patients at high risk.