The assumption that successful allogeneic BMT relies on the myeloablative effect of intensive but hazardous chemo-radiotherapy has largely restricted this therapeutic modality to patients with malignant or life-threatening hematologic disorders under the age of 55 years. Treatment-related mortality increases substantially with age, prior intensive treatment with chemo-radiotherapy, worsening performance status, and co-morbid medical conditions. An unacceptable risk of death from conventional BMT renders many patients ineligible for what may otherwise be curative therapy.