Fludarabine and 200 cGy TBI are commonly used for reduced-intensity conditioning preceding allogeneic hematopoietic SCT (HSCT). However, graft rejection and disease relapse are significant causes of treatment failure with this regimen. We modified this regimen by escalating the TBI dose to 400 cGy in 40 patients with hematologic malignancies. Thirty-four patients achieved complete donor T-cell chimerism at a median of 40 days following HSCT. The incidences of grades II-IV and III-IV acute GVHD were 40 and 15%, respectively, whereas that of limited and extensive chronic GVHD were 12 and 20%, respectively. Two patients rejected their grafts and 12 relapsed. The 100-day mortality was 18%, 2-year transplant-related mortality 20% and overall survival was 58% at a median follow-up of 16 months. There were no significant survival differences between patients with lymphoid compared to myeloid malignancies. A dose of 400 cGy TBI administered with fludarabine is well tolerated and further study is needed to determine whether outcomes are superior to those with 200 cGy TBI.