A 45-year-old male with acute lymphoblastic leukemia in first remission was successfully transplanted with G-CSF (filgrastim)-mobilized syngeneic PBPC (peripheral blood progenitor cells). Conditioning regimen consisted of high-dose etoposide, cyclophosphamide and total body irradiation. For PBPC mobilization the patient's healthy syngeneic brother was subcutaneously stimulated with 16 micrograms/kg/day G-CSF for 5 consecutive days. Only mild adverse effects (myalgia, transient changes of laboratory test) were observed. PBPC-apheresis was performed three times without complications on days 4, 5 and 6 after the start of G-CSF application. The unmanipulated PBPC (total 5.75 x 10(8)/kg mononucleated cells) were transfused immediately after each collection. Engraftment under administration of G-CSF (5 micrograms/kg/day subcutaneously) from day 0 to +12 was very rapid. Neutrophils exceeded 1.0 x 10(9)/l on day +9, platelet counts reached 50 x 10(9)/l on day +13. On day +27 the patient was discharged from the hospital with normal blood counts except mild anemia (Hb 9.5 g%). Bone marrow aspiration was repeatedly normocellular with normal differential count and no leukemia. We demonstrate that G-CSF-mobilized syngeneic PBPC transplantation after myeloblative chemoradiotherapy results in rapid and so far stable hematologic reconstitution with an actual follow-up of more than 15 months. This experience may be of relevance for future allogeneic G-CSF-mobilized PBPC transplantation.