The purpose of this study was to evaluate reticulocyte parameters by means of flow cytometric reticulocyte counting in a group of patients who had undergone autologous and allogeneic bone marrow transplantation (BMT). The pattern of reticulocyte response and the predictive value of absolute neutrophil count (ANC), platelet count, number of CD34(+) cell infused and graft source for reticulocyte response were studied. We compared absolute reticulocyte count (RetAbs), mean fluorescence index (MFI) and mean reticulocyte volume/mean corpuscular volume (MRV/MCV) ratio with conventional criteria (ANC and platelet count) in 22 allogeneic and 20 autologous BMT recipients. An abrupt increase in MRV/MCV ratio or a rise in MFI value were the earliest signs of erythropoietic recovery following allogeneic transplantation (63.6 and 22.8% of cases, respectively). In 13.6% of the cases, both parameters were observed simultaneously. All but three autologous transplant recipients showed changes in reticulocyte parameters earlier than ANC recovery. Granulocyte recovery and peripheral blood progenitor cells (PBPC) graft were predictive variables for RetAbs response in allogeneic transplant recipients. In the autologous group, predictive variables for RetAbs response were a high number of CD34(+) infused cells and platelet recovery. An increase in the immature reticulocyte population is the earliest sign of haematopoietic recovery following BMT.2514754