Highly fluorescent reticulocyte (HFR) counts and reticulocyte counts were analyzed daily using a Sysmex R-1000 automated reticulocyte counter after 42 BMT in children (median age 7 years, range 10 month-18.5 years). White blood cell levels were also analyzed daily using either traditional counting methods or, when levels were very low, cytocentrifuge preparations. Twenty-eight patients received autologous and 14 allogeneic BMT. After myeloablative therapy, HFR counts fell to zero and rose to 2% of the reticulocyte total after a median time of 10 days (range 6-23 days) post-autoBMT and 14 days (range 9-29 days) post-alloBMT. The appearance of HFR (at least 2% in two successive counts) preceded the attainment of 20 x 10(9)/l reticulocytes in 23 of 26 autoBMT by a median time of 3 days, and in 13 of 13 alloBMT with a median time of 4 days (p < 0.001). Two per cent HFR preceded the attainment of 0.5 x 10(9)/l neutrophils in the majority of BMT by a median time > 6 days (p < 0.001). The attainment of 0.05 x 10(9)/l monocytes was preceded by a rise in HFR in 12 of 26 autoBMT and in 7 of 13 alloBMT. Median times between 2% HFR and monocyte levels of 0.05 x 10(9)/l were respectively 2 (range 0- > 24) and 3 (range: 0- > 19) days after auto and alloBMT. These results show that the evaluation of erythropoiesis following BMT by means of HFR counting provides an early measure of hematopoietic reconstitution which is as precise and considerably more practical than monitoring the monocyte level.