An alphoid repetitive DNA (D8Z2) probe specific for the pericentromeric region of chromosome 8 was used to detect extra copies of chromosome 8 in bone marrow cells obtained from 10 patients with hematological disorders and five controls. Numerical aberrations of chromosome 8 were established by conventional banding techniques. Trisomy 8 was found in four patients with myelodysplastic syndrome (MDS) and three with acute myeloid leukemia (AML). Three additional patients with MDS exhibited an extra chromosome 8 in only one metaphase. In five of the seven trisomy cases, the presence of the trisomy 8 clone was confirmed by in situ hybridization (ISH). In one case of AML with trisomy 8, detected by GTG-banding, no significant numbers of cells containing three spots were found using the alphoid repetitive probe; however, hybridization with a chromosome 8-specific library revealed that the alleged extra chromosome 8 was a translocation chromosome containing only the long arm of chromosome 8. Due to a lack of material, it was not possible to achieve optimal ISH results on the trisomy 8 bone marrow cells of patient 7. In the three MDS patients with a single trisomy 8 metaphase, a slight, albeit significant, increase of trisomy 8 interphase cells was found with ISH. We conclude that this probe is useful for cytogenetic studies. Moreover, ISH, in general, is a powerful tool for precise classification of chromosomal aberrations and can also contribute significantly to the clinical evaluation of patients with hematological disorders.