The karyotype of acute myeloid leukemia (AML) blasts has a major prognostic importance. However conventional cytogenetic studies of AML patients may fail to reveal chromosome abnormalities. Trisomy 8 is a common numerical abnormality in all AML subtypes. We evaluated the role of fluorescent in situ hybridization (FISH) in the detection of trisomy 8 in de-novo AML, and compared the results to chromosome analysis in some patients. Cytogenetic studies were performed in 9 of 12 patients. In three patients no metaphases were obtained. Of the remaining six, trisomy 8 was only detected in the metaphases of 1 patient. In contrast, 4 patients showed +8 with FISH and one had a borderline value. We conclude that FISH is a rapid and sensitive method to detect numerical aberrations in AML. In the future larger prospective studies should explore the biological and clinical application of the FISH method in different hematological malignancies.