Defect in function of tumor suppressor genes may lead to initiation/progression of leukemias. RB1, CDKN2A and TP53 gene alterations are found in acute lymphoblastic leukemia (ALL) in children. Data showing a contribution of these alterations to the pathomechanism of leukemias are contradictory and their impact on a disease course still remains undefined. The main aim of the study was to identify and the characterize of RB1, CDKN2A and TP53 allele loss in ALL children patients at diagnosis. 46 children with de novo ALL were examined. Fluorescent in situ hybridization was performed on bone marrow smear preparations. We demonstrated that at least one of three investigated deletions occurred statistically more frequently in T-lineage leukemia patients (p = 0.044); this was the most frequent in respect to RB1 gene (p = 0.054). Additionally, at least one of the examined deletions was observed statistically more frequently in patients with WBC above 20 000/µl (p = 0.043), this was the most frequent for CDKN2A gene (p = 0.066). Presented results seem to give an evidence that deletions of RB1 and CDKN2A genes may contribute to the development of hyperleukocytic type of T-lineage ALL in children, nevertheless this observation needs further investigations.