Hemorrhagic cystitis (HC) is a diffuse inflammation of the bladder of an infectious or non-infectious etiology, causing bleeding of the bladder mucosa. There are no explicit guidelines defining the appropriate treatment of HC. Hyperbaric oxygen therapy (HBO) is a non-invasive method involving the use of 100 % oxygen under increased pressure, which penetrates to poorly perfused areas. The most appropriate group for treatment with HBO is patients with BK virus-associated HC after allogenic human stem cell transplantation (alloHSCT). In this report, we present five patients after alloHSCT from a matched unrelated donor with symptoms of HC successfully treated with HBO. All patients received therapy with 100 % oxygen in a hyperbaric chamber at 2.5 atmospheres for 60 min, delivered 5 days per week. Complete response with resolution of pain and hematuria, as well as eradication of viral load, was achieved by all the patients after a mean of 13 sessions (range 11-30) of HBO. These data indicate that HBO therapy is sufficient and effective in the treatment of HC, and represents a well-tolerated procedure with good clinical and laboratory results after ineffective primary treatment.