The myelotoxicity is one of the most severe adverse events of radiotherapy. Increase of CD34+ cells level in peripheral blood as result of raised output of granulocyte colony stimulating factor (G-CSF) can be result of hematopoiesis regeneration after radiotherapy. The aim of this study was to determine the hematopoiesis regeneration using analysis of CD34+ cells level in peripheral blood and serum concentration of G-CSF in patients treated with radiotherapy according to irradiated body region and irradiation field size. Two groups of irradiated patients were examined. Group I consisted of 11 patients (mean age 56) with gynecological malignancies (teletherapy dose 40-50 Gy for pelvic area and brachytherapy with Cs). Group II consisted of 10 patients (mean age 58) with head and neck malignancies (teletherapy only 50-70 Gy). Every patient was evaluated 3 times: before radiotherapy, in the day of ending and 14 days after therapy. 3 ml of blood for CD34 and serum for G-CSF estimation were collected. Blood cells were stained with monoclonal antibody specific for CD34 antigen and analysed by flow cytometry. G-CSF level was estimated by ELISA. After radiotherapy in both groups statistically significant leukopenia (p < 0.001) was observed. There was no difference between two groups in levels of CD34+ cells before and in the last day of therapy but there was significant increase of CD34+ cells in group I compared with group II 14 days after treatment (p < 0.01). Decrease of CD34+ cells during radiotherapy and after its ending in all patients was observed but only in group II was statistically significant. Positive correlation between amount of leukocytes and CD34+ cells percentage was stated. There were no statistically significant differences in serum G-CSF concentration within particular groups and between group I and II. Our results indicate that evaluation of CD34+ cells level in peripheral blood is useful in prediction of hematopoiesis regeneration after radiotherapy. G-CSF serum concentration is not prognostic factor in these groups of patients.