Radiotherapy is a well-established treatment for some solid tumours. Hyperbaric oxygenation (HBO) may improve radiotherapeutic killing of hypoxic cancer cells, so the simultaneous administration of radiotherapy and HBO may reduce mortality and tumour recurrence. We performed a systematic search of the literature in September 2007 for randomised controlled trials, and made pooled analyses of pre-determined clinical outcomes. Nineteen trials contributed to this review (2286 patients). There was a reduction in mortality for head and neck cancers at one and five years after therapy (at five years RR 0.82, P=0.03, NNT=5), and improved local tumour control at three months (RR 0.58, P=0.006, NNT=7). Any advantage is achieved at the cost of an increased rate of both severe radiation tissue injury (RR 2.35, P<0.0001, NNH=8) and the chance of seizures during therapy (RR 6.76, P=0.03, NNH=22). There is some evidence that HBO improves local tumour control and mortality for cancers of the head and neck, and local tumour recurrence in cancers of the uterine cervix. These benefits may only occur with unusual fractionation schemes. HBO is associated with significant adverse effects including oxygen toxic seizures and severe radiation tissue injury. The methodological and reporting inadequacies of the studies included in this review demand a cautious interpretation. More research is needed for head, neck and uterine cervical cancer, but is probably not justified for bladder cancer. There is little evidence available concerning malignancies at other sites. Bennett, Feldmeier, Smee, Milross, , , , , (2008). Hyperbaric oxygenation for tumour sensitisation to radiotherapy: a systematic review of randomised controlled trials. Cancer treatment reviews, 2008 Nov;34(7):577-91. https://www.ncbi.nlm.nih.gov/pubmed/18640781