Radiation's late effects may lead to complex and potentially devastating complications in normal tissue. In many cases, resulting radionecrotic lesions seriously impair form and function which may be life-threatening and more devastating than primary malignant disease. Hyperbaric oxygen therapy is being used for treatment of radionecrosis for the last twenty years and is included in indication list of two important scientific committees, Undersea and Hyperbaric Medical Society (UHMS) and European Commitee for Hyperbaric Medicine (ECHM). In this report, we present an upper outside quadrant of the breast-soft tissue radionecrosis case which treated with HBO. The efficacy of hyperbaric oxygenation was followed by LENT/SOMA scoring. After 40 sessions of daily 90 minutes of HBO therapy at 2.0 ATA (athmosphere absolute) in a monoplace pressure chamber, the LENT/SOMA scoring which was 23/1.642 (LENT score 1/2) on admission found to be 12/0.857 by the end of the therapy and 4/0.285 on sixth month control examinations.