Nonmyeloablative allogeneic peripheral blood progenitor cell transplantation with low-dose total body irradiation (TBI; 200 cGy) plus fludarabine followed by cyclosporine and mycophenolate mofetil results in modest graft rejection rates. Acute and chronic graft-versus-host diseases (GVHD) are also seen and may not differ substantially from those that occur after fully ablative transplantation. Adding antithymocyte globulin (ATG) to pretransplant conditioning produces substantial immunosuppression. Because of its persistence in the circulation, ATG can achieve in vivo T-cell depletion. Twenty-five patients who were not eligible for conventional fully ablative allogeneic stem cell transplantation by virtue of age or comorbidities underwent nonmyeloablative allogeneic transplantation with ATG 15 mg/kg/d days -4 to -1, TBI 200 cGy on a single fraction on day -5, and fludarabine 30 mg/m(2)/d on days -4 to -2. Oral mycophenolate mofetil 15 mg/kg every 12 hours and cyclosporine 6 mg/kg every 12 hours were started on day -5. Grafts were unmanipulated peripheral blood progenitor cells mobilized with filgrastim 10 microg/kg/d and collected on day 5. The median age of the recipients was 57 years (range, 30-67 years); diagnoses were non-Hodgkin lymphoma (n = 11), acute myeloid leukemia (n = 6), multiple myeloma (n = 3), acute lymphoblastic leukemia (n = 2), severe aplastic anemia (n = 1), paroxysmal nocturnal hemoglobinuria (n = 1), and myelodysplastic syndrome (n = 1). The median CD34(+) and CD3(+) contents of the grafts were 7.6 x 10(6)/kg and 1.6 x 10(8)/kg, respectively. Five patients received voluntary unrelated donor grafts. Three patients, 2 with voluntary unrelated donor grafts and 1 with a sib donor, received a 1 antigen-mismatched graft. The rest were fully matched. Twenty-two of 25 patients were evaluable for chimerism. Sixteen had >/=95% donor chimerism. Four patients displayed 80% to 90% donor chimerism, 1 displayed 78%, and 1 displayed 64%. Eleven patients relapsed with their original disease. One patient rejected the graft at 180 days. The median hospital stay was 27 days. Complications included GVHD in 6 patients (3 patients had grade I or II GVHD of skin and liver, and 3 patients had grade III or IV GVHD of liver and gut). Two of the patients with GVHD had mismatched grafts. Transplant-related toxicity was seen in 4 patients and infection in 5 patients. The median length of follow-up was 162 days (range, 17-854 days). Complete remissions were seen in 10 patients. Four patients remained in complete response (CR) at 280 to 595 days. One patient relapsed with non-Hodgkin lymphoma after a CR of 728 days. Of the 25 patients, 16 died (6 of relapsed disease, 4 of GVHD, 3 of infection, and 3 of transplant-related toxicity) and 9 are alive (6 with CR-2 of them after donor leukocyte infusion-and 3 with relapsed disease). The addition of ATG to low-dose TBI and fludarabine nonmyeloablative conditioning was well tolerated and resulted in >80% donor engraftment in this small cohort. As in other series of truly nonmyeloablative transplantation, a high rate of relapse was observed. Donor engraftment may be facilitated by the addition of ATG to low-dose TBI and fludarabine conditioning.