A 19-year-old male underwent allogeneic BMT for severe aplastic anaemia (SAA) from his HLA- and blood group-identical sister. He was conditioned with cyclophosphamide (CY) and single fraction total lymphoid irradiation (TLI). GVHD prophylaxis consisted of FK506 and a short course of methotrexate. The patient failed to achieve durable trilineage hematopoietic engraftment. There was no significant myeloid response to GM-CSF or G-CSF. Evaluation of FACS-sorted peripheral T cells from the patient by fluorescence in situ hybridization (FISH) revealed mixed chimerism (44% host origin). Fifty-three days after the first BMT, he was treated with G-CSF primed, unmanipulated PBSC transfusions (5.28 x 10(8)/kg mononuclear, 4.28 x 10(6)/kg CD34+, 292.51 x 10(6)/kg CD3+ cells) from his original donor without reconditioning. FK506 was continued at the same dose. Neutrophil recovery to 0.5 x 10(9)/l and platelet engraftment to 20 x 10(9)/l was achieved 11 and 27 days following the first dose of allogeneic PBSC transfusion, respectively. On day 23 a repeat FISH on the patient's sorted peripheral T lymphocytes revealed 91% donor origin T cells. The patient is currently well with a stable engraftment 6 months following allogeneic PBSC transfusion, with no signs of acute of chronic GVHD.