G-CSF accelerates neutrophil recovery after autologous peripheral blood progenitor cell transplantation (aPBPCT), although the optimal timing for its administration is currently unknown. In order to establish the role and the optimal timing of administration of G-CSF after immunoselected CD34+ aPBPCT, we analyzed the data from 21 consecutive patients affected by haematological malignancies. Patients were randomized into three groups according to G-CSF administration after transplantation: day +1 (group B); day +7 (group C) or no G-CSF (group A). Serum G-CSF level was evaluated until engraftment. The CD34+ cell dose reinfused was similar (P = 0.48). G-CSF significantly reduced time to recovery of PMN >0.5 x 10(9)/l (11 vs 14 vs 20.5 days) (P= 0.00046); >1.0 x 10(9)/l (12 vs 15 vs 22) (P = 0.001). No difference was observed in the number of days with PMN <0.1 x 10(9)/l (5.5 vs 7 vs 8 days). Platelet count >50 x 10(9)/l and >100 x 10(9)/l, reticulocytes >1%, length of hospitalization, non-prophylactic antibiotic therapy, fever, incidence of sepsis and transfusion support did not differ. Early or delayed G-CSF after immunoselected CD34+ aPBPCT significantly accelerated PMN recovery but did not reduce the amount of supportive treatment or the duration of hospitalization. Delaying the initiation of G-CSF did not reduce the length of treatment (11.5 vs 12 days). Early or delayed G-CSF administration resulted in G-CSF peak serum levels 7 (early)-12 (delayed)-fold greater than an endogenous response to neutropenia.