Structural abnormality of the 11q23 band (11q23+) bearing the MLL gene translocation (MLL+) is a recurrent chromosome change observed in 3% to 7% of acute lymphoblastic leukemias and in 3% to 4% of acute myeloblastic leukemias. The resolution of conventional cytogenetics (CC) in detecting 11q23 rearrangement is limited when the translocative partner has a telomeric location; furthermore, CC can barely discriminate between true 11q23+/MLL+ and rearrangements clustering within the 11q22 to approximately 25 region without MLL involvement (MLL-). We characterized a series of 378 consecutive patients with adult acute leukemia by using CC, fluorescence in situ hybridization (FISH), and multiplex karyotyping (M-FISH) analysis. Our aim was to define the frequency of cryptic MLL+ cases and the frequency of MLL+ within 11q22 to approximately 25+ cases. As expected, FISH was more sensitive than CC in detecting MLL+ cases, but rather unexpectedly, 9 (45%) of 20 patients with 11q22 to approximately 25+ were MLL-. A better characterization of 11q22 to approximately 25+/MLL- leukemias is relevant for the identification of new, recurrent translocations. Moreover, these cases should be readily distinguishable from 11q23+/MLL+ cases. We recommend that karyotypic analysis always be complemented by molecular or FISH methods to unravel MLL rearrangements.