This is a continuation of a pilot study which is now regarded as a phase II trial with a plan to enroll an additional 40 patients (20 related and 20 unrelated donor transplants) with hematological malignancy assessing the safety and efficacy of a minimally myelosuppressive regimen with pentostatin and low-dose total body irradiation (TBI) followed by allogeneic peripheral blood stem cell transplantation (alloPSCT).