An implanted Silastic catheter technique was used to measure partial pressure of oxygen in mastectomy wounds and needle-induced wounds in the subcutaneous tissue of the arms of 33 postoperative patients to assess tissue-wound oxygenation and perfusion on the day of operation and daily through postoperative day five. Characteristic patterns were observed. Wound hypoxia was common and most pronounced after abdominal, vascular, and cardiac procedures. It was most severe immediately after operation. Tissue hypoxia was not easily detected by clinical evaluation and was unknowingly tolerated by experienced surgeons. The relationship between arterial and tissue PO2 is biphasic and presumably curvilinear at the lower range of PaO2 and rises linearly even above the point of full saturation of hemoglobin. Supplemental bolus fluid infusion elevated depressed tissue PO2 in 19 out of 19 measurements, implicating hypovolemia as a common cause of postoperative tissue hypoxia. Measurements of tissue oxygen tension, coupled with a single arterial oxygen determination, constitute a clinically useful means of monitoring tissue perfusion.