This is a pilot study to evaluate the efficacy of hyperbaric oxygen therapy (HBO2) for mobilizing hematopoietic progenitor cells from bone marrow to blood. These cells are needed for patients to undergo bone marrow transplantation and some patients fail to respond to current best chemotherapy. HBO2 has been shown to trigger stem cell mobilization in other patient populations and we plan to investigate whether this intervention can act in concert with chemotherapeutic agents to allow poor mobilizer patients to achieve successful bone marrow transplantation. Twenty patients will be identified by participating hematologists who have failed to respond adequately to chemotherapy. When it is deemed appropriate to attempt an additional stem cell mobilization protocol, these patients will be administered chemotherapy as determined by their primary treating hematologist and additionally receive daily HBO2 (2.5 atmospheres absolute [ATA] for 90 minutes) for 3-8 days. At intervals, blood samples will be obtained as is the normal transplantation protocol practice to assess whether adequate stem cells are present in blood for the patient to proceed with transplantation. The project is anticipated to take one year to complete.