Follow-up data at five years are reported for 24 patients with squamous cell carcinoma of the head and neck, included in a randomised prospective MRC study of radiotherapy in hyperbaric oxygen (10 fractions) or air (15 or 20 fractions). Although few, the data show a significant gain in local control (P less than 0.001) and survival (P less than 0.05) with the use of hyperbaric oxygen but no increased tissue reactions or morbidity.