Introduction Cancer is affecting a growing number of persons. Still, the treatment and survival of cancer is improving. Radiation therapy is used in the treatment of cancer. Late radiation-induced injuries afflict 5-15% of irradiated patients. The urinary bladder and bowel may be affected after irradiation of cancer in the pelvic region. Symptoms can be severe, with impaired health related quality of life (HRQoL). Hyperbaric oxygen therapy (HBOT) involves breathing oxygen at high ambient pressure. HBOT can reverse radiation-induced injuries, alleviate patient-perceived symptoms, and improve HRQoL. We aimed to clarify the effects of HBOT on late radiation-induced injuries in the urinary bladder and bowel, and to clarify some of the underlying mechanisms through which HBOT exerts its effects. Methods A prospective cohort study assessed effects of HBOT on patient-perceived symptoms (Paper I). A rat study assessed reversal of radiation-induced stress with HBOT (Paper II). A methodological experiment assessed reversal of HBOT on cellular death induced by radiation (Paper III). A multi-center, randomized, controlled trial assessed patient-perceived symptoms, HRQoL, and objective clinical outcomes (Paper IV). Result HBOT can alleviate patient-perceived symptoms, reduce objective findings, and improve HRQoL in patients affected by late radiation-induced injuries (Paper I, IV). Oxidative stress and downstream effects, induced by the irradiation, can be reversed by HBOT (Paper II). Paper III outlines a method for studies on urothelial cells exposed to radiation and HBOT. Conclusion HBOT can reduce radiation-induced oxidative stress and inflammatory response. HBOT can reverse injuries induced by radiation therapy to the pelvic region, alleviate patient-perceived symptoms and lead to improved HRQoL.