We describe a 42-year-old man with ANLL-M4 in relapse after allogeneic BMT, in whom a new CR was obtained by conventional chemotherapy followed by the infusion of his female donor PBSC. At the time of BMT he was in CR. Six months later a full hematological relapse occurred an a three drug 5-day regimen was started. Two days after the end of chemotherapy he received donor PBSC collected by two leukaphereses after mobilization with G-CSF, given subcutaneously at 5 micrograms/kg/day for 7 days. The mononuclear PBSC were 4.2 x 10(8)/kg; the CD34 positive cells were 8.2 x 10(6)/kg and the CFU-GM were 14 x 10(4)/kg. Two days after PBSC infusion the patient received G-CSF at a dose of 5 micrograms/kg/day. Hemopoietic recovery occurred promptly on day + 13 and Y-FISH revealed 14% of Y-spot positive cells in the marrow. On day +20 hematological and cytogenetic remission was documented. The percentage of recipient cells decreased from day +36 onwards following the occurrence of a grade II GVHD, from which the patient recovered 1 week later with oral cyclosporin A and intravenous high-dose steroids. At present (day +200 from relapse) the patient is still in CR with 3% of Y-spot positive cells.