AlloPBSC-T is rapidly replacing bone marrow transplantation. The minimum number of progenitor cells required for rapid engraftment following PBSCT is unknown. 12 patients underwent alloPBSC-T during treatment for haematological malignancy. PBSC's were mobilised with Filgrastim (10microg/kg/day sc). The mobilisation was monitored daily by the number of mononuclear cells (MNC) and CD34+ cells in peripheral blood. Collections were normally performed on days 4 and 5, by means of apheresis (Fenwall CS 3000+). No further manipulation of PBSC was performed. A median of 10.15x10(8)/kg MNC (range 5.87-12.24), 9.075x10(6)/kg CD34+ (range 0.78-18.98), 53.85x10(4)/kg CFU-G (range 17.2-138.4), 28.05x10(4)/kg CFU-M (range 4.1-102.2), 115.65x10(4)/kg BFU-E (range 9.1-255.2), 3.65x10(4)/kg CFU-GEMM (range 0.3-10.4) were infused into recipients following BuCyl20 conditioning. Haematopoietic reconstitution was observed in 11 patients. The median number of days to achieve a neutrophil count of 0.5x10(9)/l was 16 (range 12-20), the platelet count of 20x10(9)/l was 12 (range 8-20) and RBC transfusion independence was 11 (range 8-20). 9 recipients developed acute GVHD (3 grade I0, 2 grade II0, 1 grade III0, 3 grade IV0). We found that the number of MNC in PBSC had no influence on the number of progenitors. All the patients who received G-CFU>20x10(4)/kg, BFU-E>60x10(4)/kg and CFU-GEMM>1x10(4)/kg experienced a rapid haematopoietic recovery. No relation was found between the number of reinfused progenitors and the appearance of GVHD.