Anemia is one of the most frequent side effects of anticancer treatment, it is also caused by disease itself. Reliable laboratory tests indicating hematological recovery after chemo- and radiotherapy are needed. Effective erythropoiesis can be monitored by quantitative measurement of reticulocytes. The amount of RNA in these cells can be assessed with flow cytometry and divided into low- (LFR), middle- (MFR) and high-fluorescence reticulocytes (HFR). This distribution is correlated with their maturation.The aim of our study was to find the most sensitive indicator of anaemia among reticulocyte subpopulations assesed by flow cytometry in children with cancer. 46 children with different neoplasmatic diseases were enrolled into the study.1) we did not find any differences between control and examined group at the time of diagnosis except for IRF, which was higher in examined group (p = 0.001); 2) IRF was lower already 2-4 days after end of chemotherapy (p = 0.03), and rised before next regimen (p = 0.0006).In conclusion we showed that IRF is not only the first sign of hematologic recovery but also very strong indicator of postchemotherapy aplasia in children with cancer and may serve as a additional parameter of bone marrow function in clinical studies.