Cytogenetic examinations were carried out in 40 children with acute leukaemia. In 18 cases karyotype changes were demonstrated, in 8 cases the karyotype was normal, in 14 cases no cell division was obtained and these patients were excluded from further analysis. In the group with karyotype changes in most cases poor prognosis indices were found, including high number of blasts, extramedullary presence of leukaemic infiltrations and FAB L2 or L3. Chromosomal abnormalities included presence of aberrations in the form of translocation, mosaicism of karyotypes with nearly tetraploidy line, accessory marker chromosomes and other anomalies. In this group 8 children died (30% of all observed cases), while in the group without karyotype changes 2 children died (about 8%). In summary the authors stress the correlation between the presence of clinical prognostic factors (risk factors) and karyotype changes and worse prognosis in cases with chromosomal abnormalities. Attention is called to the great usefulness of cytogenetic examinations in the prognosis and treatment of leukaemias in children.