Leukemias are the most common malignancy in children, and acute lymphoblastic leukemia (ALL) accounts for 85% of all childhood leukemias. Sequential monitoring of MRD (minimal residual disease) in a set time points during the induction therapy in ALL proves to be a powerful and independent predictor of treatment outcome. Crucial limitation of MRD monitoring by flow cytometry is immunophenotypic changes seen at relapse. Up to now there are single publications about immunophenotypic changes during treatment of ALL in children. Objective was to assess changes in expression of cell antigens during induction treatment of ALL. From May 2005 to January 2008, from among 78 patients with ALL treated in Oncology and Hematology Department, Children's University Hospital in Krakow according to international treatment protocol ALL IC-BFM-2002, 42 were enrolled in assessment of antigens' cells modulation expression during induction. Finally, 24 boys and 18 girls were eligible for evaluation. For MRD detection 4-colour flow cytometry with FACS Diva Software v.5.1 (BD Immunocytometry Systems) was used. The panel of monoclonal antibodies used for MRD detection was based on ALL IC-BFM-2002 standard, modified by additional antibodies combinations from ALL-BFM-2000 protocol. Identification of leukemia associated phenotype (LAP), used for cell analysis in sequential time points, with a set monoclonal antibodies panel, was possible in all analyzed patients. Superficial and cytoplasmic antigens modulation was observed in most of the patients during MRD monitoring. Changes of antigens were seen mostly in PB on day 8 and on day 15 both in PB and BM. The most common antigen modulation found in children with cALL was: downmodulation of CD10, CD34, CD19, and upmodulation of CD45, CD11a, CD20. Unequivocal character of modulation of CD66c, CD58, CD38 was difficult to define (up and downmodulation). Introduction of treatment monitoring based on flow cytometry MRD measurement could lead to further individualization of therapy and improovement of cure rates in ALL, as well as to reducing side effects of ALL treatment and decrease total cost of patient therapy.