Background. It was found by classical cytogenetic methods that approximately 80 % of X-chromatin positive males have karyotype 47,XXY and the rest have mosaicism of sex chromosomes. More sensitive molecular-cyto- genetic studies allow to evaluate even non-dividing cells of different tissues and therefore they suggest that there could be more frequent occurrence of patients with numerical changes of gonosomes than quoted in the literature. The aim of this study was to detect numerical changes of sex chromosomes by means of double-colour fluorescence in situ hybridisation (FISH) in dividing and non-dividing nuclei of peripheral lymphocytes of males with Klinefelter’s syndrome, to compare clinical findings with cytogenetic results, to determine differences in sensitivity of classical and molecular-cytogenetic methods and estimatere the values obtained with controls (healthy males). Methods and Results. 26 males with previously diagnosed Klinefelter’s sy were examined. Classical cytogenetic studies consisted of evaluation of at least 20 G-banded mitoses of 72h cultivated peripheral blood. The results we obtained: 19 patients with 47,XXY, 5 mosaics 47,XXY/46,XY, 1 patient mosaic 46,XX/47,XXY and 1 patient 48,XXYY karyotype. The results of double colour FISH and classical cytogenetics were compared and mosaics of cells with normal karyotype 46,XY was found. As a control 10 healthy males were examined and mosaics of gonosomes were not detected. Conclusions. FISH method has a higher sensitivity for detection of sex chromosomes mosaics than classical cytogenetics. The existence of small cellular side clones with 46,XY karyotype or numerical sex chromosome changes which were not determined previously can be proved by FISH in patients with Klinefelter’s sy.

        Key words: Klinefelter’s sy., FISH, numerical changes of X and Y chromosomes, interphase cell.