Chromosomal aberrations affecting the tumor cell are frequently specific for a certain type of tumor. The demonstration of them is applied in clinical practice to confirm the diagnosis, in an atypical histological fin-ding, unusual clinical course, or for prognostic histological finding, unusual clinical course, or for pro-gnostic estimates. They are detected by examining the caryotype, by fluorescence in šitu hybridization (FISH) or by its variants - multicolor FISH (mFISH), spectral caryotyping (SKY), or comparative genomic hybridization (CGH). Examples are the verification of a peripheral primitive neuroectodermal kidney tumor by the demonstration of a translocation, t( 11 ;22), specific for the tumor, and the specification of the dia­gnosis of a germinal tumor of the mediastinum with malignant transformation into a rhabdomyosarcoma by the demonstration of amplified 12p genetic materiál. Demonstration of the Philadelphia chromosome, t(9;22), is important in the diagnostics of chronic myeloid leucemia. Cytogenetic investigation is also essen-tial in the classification of the myelodysplastic syndrome (MDS) in as certain forms of that disease are defi-ned on the basis of chromosomal aberrations. Prognosis in certain tumors is connected with the presence of specific chromosomal aberrations. Another important indication is the search for signs that allow the demonstration of a minimal tumor disease. Molecular cytogenetics methods represent established diagnos-tic procedures having a wide spectrum of applications in clinical oncology. Such investigations háve beco-me a component of a number of diagnostic and therapeutic protocols, and that should be kept in mind in the examination of a patient suspect of a malignancy. Last but not least, at present there is being introduced into practice antitumor therapy focused against specific cellular molecules demonstrated námely by molecular cytogenetic investigation.

        Key words: malignant tumors - chromosomal aberrations - caryotype - fluorescence in šitu hybridization -comparative genomic hybridization.