Background: This study investigated the effects of hyperoxic treatment on growth,angiogenesis, apoptosis, general morphology and gene expression in DMBA-induced ratmammary tumors.Methods: One group of animals was exposed to normobaric hyperoxia (1 bar, pO2 = 1.0 bar)and another group was exposed to hyperbaric hyperoxia (1.5 bar, pO2 = 1.5 bar). A third groupwas treated with the commonly used chemotherapeutic drug 5- Fluorouracil (5-FU), whereasanimals housed under normal atmosphere (1 bar, pO2 = 0.2 bar) served as controls. Alltreatments were performed on day 1, 4, 7 and 10 for 90 min. Tumor growth was calculated fromcaliper measurements. Biological effects of the treatment, was determined by assessment ofvascular morphology (immunostaining for von Willebrandt factor) and apoptosis (TUNELstaining). Detailed gene expression profiles were obtained and verified by quantitative rtPCR.Results: Tumor growth was significantly reduced (~57–66 %) after hyperoxic treatmentcompared to control and even more than 5-FU (~36 %). Light microscopic observations of the tumor tissue showed large empty spaces within the tissue after hyperoxic treatment, probablydue to loss of glands as indicated by a strong down-regulation of glandular secretory proteins. A significant reduction in mean vascular density (30–50%) was found after hyperoxic treatment.Furthermore, increased apoptosis (18–21%) was found after hyperoxic treatment.Conclusion: Thus, by increasing the pO2 in mammary tumor tissue using normobaric and moderate hyperbaric oxygen therapy, a significant retardation in tumor growth is achieved, byloss of glands, reduction in vascular density and enhanced cell death. Hyperbaric oxygen should therefore be further evaluated as a tumor treatment.