The role of flow cytometric reticulocyte (RET) counting and the immature RET fractions (IRF) in the evaluation of hematopoietic recovery following chemoradiotherapy-induced aplasia was studied. RET counts and IRF were studied using an automated flow cytometric reticulocyte counter (Sysmex R-2000) in three groups of patients: 58 patients undergoing an autologous bone marrow transplantation (ABMT group), 28 of whom received granulocyte colony-stimulating factor (G-CSF); 28 patients undergoing an allogeneic bone marrow transplantation (BMT group); and 28 patients receiving remission-induction chemotherapy for acute leukemia (CHEMO group). To evaluate the IRF the percentages of RET fractions with middle and high fluorescence reticulocyte (MFR and HFR, respectively) were used. A rising IRF (expressed as the percentage of MFR + HFR) was the first sign of hematopoietic recovery (ABMT group, IRF 9 days versus 18 days for the absolute neutrophil count (ANC); BMT group, 15 versus 18 days; CHEMO group, 9 versus 11 days). When recovery of the ANC (> 0.5 x 10(9)/l) was compared with that of the iRF (MFR + HFR > 5%), statistically significant differences were found in all three groups. Additionally, 93.1% of the ABMT, 92% of the BMT and 91.2% of the CHEMO recovered the IRF before the ANC. In conclusion, an elevation in the percentage of IRF is the first sign of hematologic recovery in the majority of patients receiving remission-induction chemotherapy and the first sign of engraftment in those submitted to ABMT or BMT. Serial automated flow cytometric quantitative reticulocyte counting provides a useful and early measure of erythropoiesis indicative of hematopoietic reconstitution or successful bone marrow engraftment following marrow transplantation.