Abstract Cytogenetic rearrangements were studied in bone marrow cells of 24 patients with relapse of acute lymphoblastic leukemia (ALL). The authors have noted a high percentage of mosaic karyotypes (75.0%) with a predominance of abnormal clones combined with normal karyotypes (33.3%). Trisomies of chromosomes led to the formation of hyperdiploid clones, among which trisomies of chromosomes 6, 21, 15, and 5 were the most frequent events. Additional marker chromosomes were recorded in 22.2%. To characterize the mechanisms underlying the abnormal clone formation in relapse of ALL, structural chromosomal anomalies were recorded and divided into balanced (translocations, inversions) and imbalanced (deletions, isochromosomes, additional material of unknown origin, duplications, and imbalanced translocations) abnormalities. Losses of genetic material (deletions) (24.1%) and translocations (33.3%) were most frequently detected. The total number of events with translocation t(9;22)(q34;q11.2) reached 20.4%. The evolution of clonal chromosomal abnormalities occurred due to the emergence of additional numerical and imbalanced structural abnormalities. The comparison between chromosome abnormalities at diagnosis and in relapse of B-cell ALL has not led to any general mechanisms for the formation of chemotherapy-resistant clones. Trisomy of chromosome 5, deletion del(6)(q23), and translocation t(9;22) (q34;q11.2) were involved in the formation of chemotherapy-insensitive clones. The group of unfavorable prognosis included 95.8% of karyotypes.