Work in an animal cancer model suggests that pretreatment with hyperbaric oxygen can improve tumor vascularity rendering chemotherapy more effective. Accordingly 32 subjects with locally advanced breast carcinoma (>5cm diameter) entered into a randomized clinical trial where a course was administered of six intravenous pulses of cyclophosphamide 1000mg/m2 i.v., doxorubicin 50mg/m2 i.v. and vincristine 1.5mg/m2 i.v. In the case group this was preceded by ten, once daily, sessions of hyperbaric oxygen therapy (HBO2) administered either at 2.4 or 2.0 atmospheres absolute. Eleven out of 15 subjects tolerated a full course of HBO2 and chemotherapy. All 17 control subjects tolerated a full course of chemotherapy. Tumor extravascular extracellular or edema fluid was reduced after HBO2 but there was no reduction in tumor cell volume and no indication of increased vascularity on MRI. Clinical and pathological responses to chemotherapy were the same in both groups and there was no evidence of neovascularisation. Five year survival in those who tolerated the trial regime was 73% and did not differ between the groups. This mortality was cancer related.