ALL patients with a hyperdiploid karyotype of more than 50 chromosomes (high hyperdiploidy) carry a better prognosis in contrast to patients presenting with other cytogenetic features, and an appropriate less intensive therapy protocol should be developed for these patients. For this reason it is desirable to have a quick screening method identifying those with this type of hyperdiploidy. We therefore studied the bone marrow and/or blood cells of 278 children with ALL using double target fluorescence in situ hybridization (FISH) on interphase. A combination of DNA probes (repetitive, centromere specific) was applied detecting chromosomes which are most frequently overrepresented in patients with hyperdiploidy (>50), at chromosomes 6, 10, 17 and 18. All patients showing hybridization signals differing from the normal signal distribution of two spots for each tested chromosome were analyzed cytogenetically as well. 102 children (102/278; 36.7%) were found to have a clone with aberrant FISH results. In 80 patients (80/278, 28.8%) the cytogenetic analysis detected a hyperdiploid karyotype >50 chromosomes, whereas the remaining patients (n=12) could be related to other ploidy subgroups, ie hyperdiploidy with 47-50 chromosomes, haploidy, triploidy/tetraploidy. Comparison of the FISH results with the measurements of the DNA content showed good agreement for 88.8% (208/234) of the investigated patients. The detected rate of 28.8% patients with a high hyperdiploid karyotype in our investigated cohort is comparable to the frequency of other studies. Only one patient was not identified as having a hyperdiploid karyotype with our combination of DNA probes. Our results indicate that FISH is a feasible and quick screening method for the detection of hyperdiploid karyotypes (>50 chromosomes) and other ploidy subgroups.