Translocations involving chromosome band 11q23 are frequently found in infant acute leukemia and involve rearrangement of the MLL gene. In this study, 29 cases of adult acute myeloid leukemia (AML) were analyzed to determine the frequency of MLL gene rearrangement. Of these, 19 cases were karyotyped and none showed cytogenetic evidence of 11q23 aberration. MLL rearrangements were demonstrable in four cases, giving a frequency of 14% (4/29). Only one of the four cases with MLL rearrangement showed features typical of leukemia with 11q23 aberration; other cases were indistinguishable from those without MLL rearrangement. There was no apparent difference in presentation blast count, remission, and survival duration when cases with or without MLL rearrangement were compared. Clinicopathologic features of adult AML with MLL rearrangements may be heterogeneous.