Cellular hypoxia is a cause of radioresistance. The oxygen tension (pO2) in normal tissues and in tumours can be measured by polarography. In this feasibility study we have measured the tissue pO2 of 10 patients suffering from uterine cervix carcinoma, using the Eppendorf histograph. The measurements were performed at the time of the brachytherapy after external radiotherapy. The machine was found to be reliable and no adverse effect was noted. The mean pO2 values in tumours were lower than those of normal tissues.