In microvascular tissue transfers, it is essential postoperatively to follow-up on the perfusion of the transferred flap because of the risk of anastomotic failure. The diagnosis of pedicle obstruction is usually made by clinical observation, but some techniques have been reported as more reliable than clinical observation in detecting perfusion failure. The authors used microdialysis (MD), a method developed to assess in situ tissue metabolism, in the follow-up of 80 consecutive microvascular flaps from October, 2001 to October, 2003. Of the 78 flaps with postoperative data, 58 flaps were uneventful clinically and using MD, and served as the reference material for normal postoperative metabolism. Twenty flaps showed some abnormality in the clinical course or with MD. Of these, 13 flaps were reoperated for anastomosis thrombosis (9 arterial, 4 venous). All thromboses were clearly recognized by MD via a decrease in the glucose concentration in the tissue (< 2.7 mmol/l) and an increase in the lactate concentrations (> 5.7 mmol/l). In some cases, MD indicated a pathological trend in glucose and lactate concentrations hours before there were any clinical signs. A system of alarm levels was developed for the staff: when the limits were reached, a critical evaluation of the situation was undertaken, and the need for reoperation was considered. In the series, the salvage rate of all thrombosed flaps was 77 percent, with a final success rate in microvascular reconstruction of 95 percent. No flap was lost due to a delay in the diagnosis of secondary ischemia, if on-line MD monitoring was available. Microdialysis is a clinically feasible and sensitive monitoring method for all kinds of microvascular flaps, especially for those in which clinical observation is difficult or impossible. The performance of the analysis is easy and can be done by even less experienced nursing staff working in institutes with a low frequency of microsurgery.