HBO pretreatment of UCBT is safe and may improve engraftment. A pilot clinical study was performed, with the primary end point of assessing the safety of HBO pretreatment of UCBT and secondary end points of evaluating the rates and kinetics of neutrophil and platelet engraftment. HBO pretreatment of UCBT was found to be safe and feasible. Neutrophil recovery averaged only 6 and 9 days for patients subjected to myeloablative and reduced-intensity conditioning, respectively, compared with 17 and 25 days for controls. One hundred percent of patients displayed platelet recovery by 180 days posttransplant, compared with only 69% of controls (P = .013). One hundred percent of HBO-treated patients survived 100 days posttransplant compared with only 76% of controls (P = .051).