BACKGROUND AND OBJECTIVES: In acute leukemias, chromosomal translocations involving the 11q23 band are frequently, but not invariably, associated with MLL gene rearrangement and their finding is associated with a poor prognosis. We observed two new translocations with a breakpoint in the 11q23 region at standard cytogenetic analysis: a previously undescribed t(3;11)(q21;q23) in a 70-year old woman with a fulminating form of AML-M1 and a new translocation t(6;11)(q15;q23) in a 61-year old man with an atypical chronic myelogenous leukemia. In these two patients, involvement of the MLL gene was analyzed by molecular cytogenetic techniques which also allowed a more precise mapping of the breakpoints. DESIGN AND METHODS: The MLL gene was analyzed by Southern blot and by fluorescent in situ hybridization (FISH) with a double-color MLL probe. A panel of 11q, 3q and 6q cosmid/YAC probes mapping around the breakpoints was used for breakpoint mapping. RESULTS: In both patients, FISH analysis and Southern blot showed that the MLL gene was not rearranged; in patient 1, MLL was retained on the 11q+ derivative, whereas in patient 2 it moved to the 6q- chromosome. In the t(3;11) we localized the chromosome 11 breakpoint at 11q23.3, in a region flanked by CP-939H3 and cos1p3, distal to the MLL locus; in the t(6;11) the break occurred at 11q21, in a region flanked by CP-819A5 and CP-829A6, proximal to the MLL locus. INTERPRETATION AND CONCLUSIONS: Our cases add two new translocations to the list of chromosomal anomalies involving the long arm of chromosome 11, and show that apparent translocation t(11q23) may involve loci and genes other than MLL. Characterizing the molecular heterogeneity of 11q23 translocations may identify some prognostic significance.