The clinical trials of the use of hyperbaric oxygen in radiotherapy conducted by the Medical Research Council's working party have shown that local cure and survival can be improved in head and neck and cervical cancer. Evidence for benefit has been presented in carcinoma of bronchus, but none has so far been found in carcinoma of the bladder. Although hyperbaric oxygen has produced increased effect upon normal tissues as well as upon tumour, a real improvement in therapeutic ratio appears to be present. The future use of hyperbaric oxygen in radiotherapy will depend upon a comparison of effectiveness with other methods now being tested to improve radiotherapy. The trials have yielded much fundamental and clinical data of general relevance in the radiotherapy of malignant disease.