BACKGROUND Experimental data have shown that hypoxic areas in tumors can increase their malignant potential and reduce their sensitivity to chemotherapy and radiation treatment. Until now, the only incomplete data on the oxygenation of human tumors during therapy have been available. METHODS The distribution of partial pressure of oxygen in malignant head and neck processes was measured in four patients with lymph-node metastases by means of invasive computerized histography (system manufactured by Eppendorf). All patients received primary radiation treatment and chemotherapy with a two-week respite after 30 Gy and the first cycle of chemotherapy. The total dose consisted of 70 Gy in the tumor and two cycles of chemotherapy. Oxygen measurements were taken before the onset of treatment, before and after the respite, and after completion of treatment. RESULTS In three of the four patients, we measured prominent hypoxic areas in the cancers with PO2 values less than 5 mmHg. The average value was between 12 and 46 mmHg. We were impressed by the observation that oxygenation appeared to improve in every cancer during the respite. Only the patient who showed good initial oxygenation without any measurable hypoxic fraction suffered a complete recurrence six weeks after the completion of therapy. CONCLUSIONS Prominent areas of hypoxic tissue are present in human head and neck cancers. Radiation treatment and chemotherapy can alter the oxygenation. Further studies are required to investigate the significance of the oxygenation of human head and neck cancers and the changes occurring in it during different types of therapy in order to assess its clinical impact.