Pentostatin (deoxycoformycin) is a purine analogue that is currently indicated for the treatment of chemo-naïve or interferon-refractory hairy cell leukemia. Pentostatin represents an ideal agent for conditioning recipients before allogeneic hematopoietic cell transplantation (HCT) due to its significant effect on lymphocytes and relatively reduced myelosuppression as compared to other purine analogues. It is a potent inhibitor of the enzyme adenosine deaminase (ADA) whose activity is generally more pronounced in T lymphocytes as compared to their B counterparts. Also, pentostatin has been shown, though its immunomodulatory effect to be active in patients with steroid-refractory acute graft versus host disease (aGVHD).