Although there have been some reports that hyperbaric oxygen therapy (HBOT) is effective in treating breast cancer-related lymphedema (BCRL), controversy regarding its therapeutic effects remains.We sought to evaluate the efficacy of HBOT in addition to conventional complex decongestive therapy (CDT) for BCRL.A prospective observational study was conducted on 10 patients with BCRL. After screening, the subjects were stratified into a CDT-only group and a CDT and HBOT combination (CDT-HBOT) group. All patients received a total of 10 treatments over 2 weeks. Changes in the circumference of the upper limbs, quality-of-life questionnaire results, and bioelectrical impedance values were compared between the 2 groups.Between both groups, there were no significant differences in demographic or clinical characteristics and in the quality-of-life outcomes for lymphedema of the limbs. The parameters measured by bioimpedance spectroscopy showed more significant improvements in the CDT-HBOT group than in the CDT-only group.In patients with BCRL, HBOT may be recommended as an adjunct treatment to the existing therapies.