MLL gene rearrangements identify a subgroup of acute leukemias with a bad prognosis. Fifty-one adult patients with de novo AML were screened for MLL rearrangements by Southern blot. An unexpected high frequency of AML-M2 according to the FAB classification was found. The most striking morphologic features of these cases were the scarcity of Auer rods and granulocytic dysplasia. Morphologic traits could be helpful in distinguishing molecular groups of AML-M2.