Fluorescence in situ hybridization analysis was carried out in five patients with acute myeloblastic leukemia of various French-American-British subtypes and with double trisomy of chromosomes 8 and 21. PML-RARA fusion was detected with appropriate molecular probes in one patient with acute promyelocytic leukemia without t(15;17). Two PAC probes covering the 5' and 3' part of the RUNX1 gene were used in the four other patients. While three copies were present in three patients, as expected from conventional karyotype analysis, only two hybridization signals were present in the fifth patient. This was due to the apparent loss of the 3' part of RUNX1. Since chromosome number abnormalities may be associated with submicroscopic gene rearrangements, it should be important to search for them for a better understanding of mechanisms of leukemogenesis, and to understand the prognostic heterogeneity in leukemic patients with aneusomies without apparent chromosome structure rearrangements.