Clinical examination is still the gold standard of postoperative free flap monitoring, but with intraorally situated and/or buried flaps, it can be difficult or impossible. Microdialysis is a sampling technique which offers the possibility to monitor the metabolism of a flap continuously. Ischemia can be detected by monitoring the changes in glucose, lactate, and pyruvate levels in interstitial fluid of the specific tissue. Our aim was to use microdialysis to monitor the metabolism of free flaps used for reconstructions inside the oral cavity/oropharynx and to evaluate the reliability and usefulness of this new monitoring method.Twenty-five consecutive patients who underwent oral cavity/oropharynx cancer resection and immediate reconstruction with free flap were included in the study. A microdialysis catheter was placed into the subcutaneous adipose tissue of the flap in the end of the surgical procedure. Dialysate samples were taken on an hourly basis for 72 hours postoperatively. Routine clinical monitoring was carried out by experienced nursing staff. Clinical findings were recorded and later compared with microdialysis values. Two flaps out of 25 failed in spite of reoperations. In both problem cases, microdialysis indicated ischemia 1 to 2 hours before it became clinically evident. During flap ischemia, the lactate/pyruvate ratio increased, glucose concentrations reduced, whereas lactate level increased when compared with normal values. Our results indicate that microdialysis is safe for the patient and the flap. It can reliably detect flap ischemia at an early stage. This is especially useful in buried flaps when clinical monitoring is difficult. Microdialysis may also reduce the patient discomfort caused by repeated clinical examination of the flap.