Cytogenetic analysis of one case of acute myeloid leukemia (AML), one of acute lymphoblastic leukemia (ALL), one of refractory anemia with excess of blasts (RAEB), and one of acute mixed lineage leukemia (AMLL) with unbalanced 7;12 translocations mapped the breakpoints to the centromeres on both chromosomes. The rearrangements were interpreted as the whole-arm translocations der(7;12)(q10;q10) in the AML and ALL and der(7;12)(p10;q10) in the RAEB and AMLL. However, further analysis by metaphase and/or interphase fluorescence in situ hybridization (FISH) showed centric fusion only in the AML and ALL. In the RAEB and AMLL, centromeric material from chromosome 7 but not from 12 was present in the derivative chromosome. Whereas the t(7;12) resulted in loss of 12p in all four cases, the corresponding chromosome 7 imbalances differed--monosomy for 7q in the RAEB and AMLL and monosomy for 7p in the AML and ALL. Six hematologic neoplasms with unbalanced whole-arm or near-centromeric 7;12 translocations and seven dic(7;12) with juxtacentromeric breakpoints have been reported previously: 2 AML, 1 RAEB in transformation, and 10 ALL. All karyotypically informative cases had loss of 12p material. All but one of the cases with combined 7p and 12p deletion were ALL, whereas all cases with 7q and 12p loss showed myeloid differentiation. No particular clinical, morphologic, or immunophenotypic features seem to characterize ALLs with t(7;12). AMLs with an unbalanced t(7;12), often together with 5q deletions, might be associated with previous genotoxic exposure and poor prognosis.