An implantable and disposable tissue oxygen tension (Po2) probe, useful as a clinical monitor following free-tissue transfer, is described. Experimentally, using the rabbit epigastric groin flap model, we have found it to be a sensitive indicator of arterial and venous occlusion. Oxygen can be given to verify the integrity of the monitoring technique and check any abnormal reading. Animals administered oxygen will have a rapid increase in tissue Po2when the pedicle is intact. If compromised by arterial or venous occlusion, the low readings demonstrate no change. The application of this new technique for monitoring free flaps in 12 patients is presented. In 2 patients the tissue Po2monitor successfully detected early vessel thrombosis, resulting in reexploration of the anastomoses and salvage of the free flaps.