Methodological prerequisites for the culture of leukaemic blasts and the fluorescence banding of spread metaphases are represented in detail. By using an improved method the percentage of aneuploid karyotype findings increased in AML from 49% to 68% and ALL from 65% to 81% of all evaluable patients. Specific aberrations known up to the present day are discussed. They could be diagnosed in 46% of aneuploid AML-clones and in 59% of ALL-clones. The fluorescence banding according to the three-colour technique is demonstrated by three examples of "non-specific" aberrations: a) Tandem translocation of 1q and 7 q (64a, male, FAB:M5B), b) Karyotype instability with variability of chromosome numbers arising from trisomy, tetrasomy and polyploidisation of single normal as well as marker chromosomes (57a, female, FAB:M6), c) Identification of a marker chromosome as a derivative chromosome 16 (33a, female, FAB:L2, c-ALL).