No serum marker has consistently proved helpful in diagnosing early acute mesenteric ischemia. D(-)-lactate is a product of bacterial metabolism and was found to be elevated in peripheral blood in a rat model of intestinal ischemia. We conducted a prospective study to evaluate preoperative D(-)-lactate levels in 31 patients undergoing laparotomy for acute abdominal emergencies, including suspected acute mesenteric ischemia. The serum was deproteinated and D(-)-lactate concentration was determined by a spectrophotometric assay. A control group was composed of patients with a benign abdominal examination who were operated on for central venous line placement. We found significant elevations in D(-)-lactate levels in patients with mesenteric ischemia compared with controls (P < 0.00005), as well as in patients with other forms of abdominal catastrophes (P < 0.00005) and with bowel obstruction (P < 0.0005). Sensitivity and specificity were 90% and 87%, respectively. The negative predictive value was 96%, and the positive predictive value was 70%. We have found that D(-)-lactate serum levels can aid in diagnosing acute mesenteric ischemia.