Between 1995 and 1997 twenty two patients with different hematological diseases ( CML n=10, AML n=6, ALL n=l, NHL n=3, SAA n=1,solid tumor n=1 ) and a median age of 37 (range, 20 to 55) years received unmanipulated peripheral blood stem cell (PBSC) transplants from HLA-identical sibling donors at our institution. Myeloablative chemotherapy consisted of cyclophosphamide (CY) and total body irradiation in 11, and chemotherapy alone in 11 patients. For graft-versus host-disease (GVHD) prophylaxis all patients were given cyclosporine A and methotrexate according to the Seattle protocol. PBSC were mobilized by granulocyte colony-stimulating factor (G-CSF) given at 10 microg/kg body weight (b.w.)/day for four days. Harvest of PBSC was started on day 5 and continued on day 6 if necessary. A median of 1 leukapheresis (range, 1 to 2) was performed and a median of 5.7 x 10(6) CD34+cells/kg b.w. (1.34 to 21.5) were obtained. Ten patients received G-CSF (5 microg/kg b.w.) starting on day one after PBSCT until neutrophil recovery. Absolute neutrophil counts >0.5 x 10(9)/L and ANC >1.0 x 10(9)/L were reached after a median of 13 (range 8 to 18) and 15 (range 9 to 19) days after PBSCT. Unsupported platelet counts >20 x 10(9)/L and 50 x 10(9)/L were reached after 17 (range 8 to 32) and 22 (range 13 to 40) days after PBSCT, respectively. Incidence of acute GVHD grade I to IV was 52%, extensive chronic GVHD occurred in 25% of patients. After a median observation time of 11 (range, 3 to 34) months twelve patients (55%) are alive and well. In summary, infusion of allogeneic PBSC after myeloablative therapy allows rapid and sustained hematologic reconstitution. Incidence of acute GVHD is not increased, for assessment of chronic GVHD longer observation times and larger patient numbers are required.