To participate in this study, subjects must have diagnosed chronic Graft versus Host Disease that is refractory to therapy or that is considered high risk (i.e. low platelet count, progressive onset and greater than 50% of body surface area affected). Subjects must have not failed more than 2 immunosuppressive regimens in order to be considered for this trial. Eligible subjects will receive intravenous pentostatin every 2 weeks for 24 weeks. If the subject has had a complete response, the therapy will end at 24 weeks. If the subject has had a partial or mixed response or stable disease, they will continue on study receiving pentostatin for 52 weeks.