In a randomized controlled clinical trial of hyperbaric oxygen in the radiotherapy of advanced carcinoma of the uterine cervix a total of 320 cases were contributed by four radiotherapy centres in the United Kingdom. The use of hyperbaric oxygen resulted in improved local control and survival. The benefit was greatest in patients under the age of 55 who presented with stage III disease. There was a slight increase in radiation morbidity but it seemed that the benefit of hyperbaric oxygen outweighed this increase in morbidity and that there was a true improvement in the therapeutic ratio.