The free intestinal flap has become a recognized part of the surgical armamentarium for the reconstruction of the cervical esophagus and in the treatment of severe short bowel syndrome. However, the intestinal flap is difficult to monitor postoperatively and is susceptible to ischemia. Entire avoidance of neglected ischemia and false alarms require a monitoring system with sensitivity and specificity of 100%. The aim of this study was to investigate the value of microdialysis (MD) as a monitoring method for detecting ischemia in intestinal transplants. In 12 pigs the entire small intestine was divided into three segments, each isolated on a vascular pedicle consisting of one artery and one vein. For metabolic monitoring of the intestinal segments, one CMA 63 MD catheter was placed in each segment in the mesentery just at the border of the intestinal wall. After 1 h of arterial ischemia followed by 2 h of reperfusion, the three intestinal segments in each pig were allocated to arterial ischemia, venous ischemia, or no ischemia. A total of 10 control segments, 10 segments with arterial ischemia, and nine segments with venous ischemia were provided for evaluation of metabolic changes. One hour of secondary ischemia induced considerable metabolic changes, with a decrease in the concentration of glucose (C (Glucose)) followed by an increase in the concentration of lactate (C (Lactate)) as well as in the lactate:pyruvate (L/P) and lactate:glucose (L/G) ratios. The changes became even more pronounced after 1(1/2) h when the L/P and L/G ratios had increased 9 and 30 times, respectively, in the ischemic segments and without overlap in values between the ischemic and the nonischemic segments. When using C (Glucose) < 0.2 mmol/l or L/G > 50 as cutoff levels for detection of ischemia, a sensitivity and a specificity of 100% could be achieved. An increase in C (Glucose) of more than 2 mmol/l, after the infusion of glucose, could be used as a challenge test to exclude ischemia. A monitoring system based on the determination of the C (Glucose) and C (Lactate) by using microdialysis can be used for positive differentiation between ischemic and nonischemic intestinal segments.