A 41-year-old female patient undergoing cyclophosphamide therapy for ovarian tumour suffered from haemorrhagic cystitis. Hyperbaric oxygenation (HBO) (100% of oxygen concentration at 2 atmospheric absolute pressures for 60 minutes a day for 30 days) was performed. After treatment the symptom subsided and haematuria disappeared. Cystoscopic findings demonstrated marked improvement. During the course of therapy no side effect was noted. Thus, HBO treatment appeared to have a beneficial effect on cyclophosphamide cystitis.