The study was performed to assess the effect of accelerated radiotherapy on oxygenation of primary tumours and metastatic nodes in patients with advanced head and neck tumours. In 14 patients with head and neck tumour, oxygen tension (pO 2 ) was evaluated in normal tissues and tumours (primary tumour or metastatic neck node) before (0 Gy) and after 2 weeks (32 Gy) of accelerated radiotherapy (70 Gy in 3.5 weeks, with three daily fractions). Radiotherapy was combined with carbogen breathing in 5 patients. pO 2 was measured using a polarographic technique. For pooled normal tissues, median pO 2 was 38mmHg before treatment and 46mmHg after 2 weeks. For tumours, very low values (<2mmHg) represented 20% of the recorded values before treatment and 10% after 2 weeks. The relative increase in tumour oxygenation was more pronounced for primary tumours (median pO 2 12mmHg before treatment versus 26mmHg after 2 weeks, P<0.05) than for metastatic nodes (respectively, 20 and 27mmHg P=0.1). For the 5 patients who breathed carbogen during accelerated radiotherapy, the median pO 2 was 44mmHg at 2 weeks, compared with 13.5mmHg before treatment (P=0.05). Very low pO 2 values, corresponding to tumour hypoxia, were found in the tumours (primary and metastatic neck nodes) prior to accelerated treatment. During the first 2 weeks of accelerated treatment, an increase in median pO 2 was found in nine of the 14 tumours, together with a decrease in the frequency of very low values.