Increased knowledge about changes that occur in tumour oxygenation during radiotherapy and the biological factors causing these changes can be useful in the development of optimal radiation treatments. The aims of this study were a) to study changes in the oxygen tension (pO2) of human head and neck tumours during radiotherapy in relationship to changes in cell density and vascular density, and b) to investigate whether the pO2, measured before or during therapy, can be used to predict the therapeutic outcome. Preliminary data from the first 11 patients included in the study are reported. The pO2 was measured before treatment (11 patients) and once a week during therapy (8 patients), using polarographic needle electrodes. Cell density and vascular density were determined from biopsies taken after each pO2 measurement in 5 patients. Significant fluctuations in pO2 occurred during therapy. Changes in hypoxic fraction; i.e., fraction of pO2 readings below 2.5 mm Hg, 5 mm Hg or 10 mm Hg, coincided with changes in cell density, but not with changes in vascular density, which suggests that the changes in hypoxic fraction were caused by changes in oxygen consumption rather than supply. Response evaluation after a median follow-up time of 19 months showed that progressive disease occurred among the patients with highly hypoxic tumour, regardless of whether hypoxic fraction before treatment or after two weeks of radiotherapy was considered.