Partial duplication of the MLL gene is a recently characterized novel genetic mechanism for leukemogenesis. A close association with trisomy 11 has been observed. A case of acute myelogenous leukemia (AML) M6 without karyotypic abnormality was found to have rearrangement of the MLL gene. Southern analysis with an MLL exon 2 probe revealed partial duplication of the MLL gene, with a duplicated MLL transcript amplified by reverse transcription polymerase chain reaction (RT = PCR). The presence of trisomy 11 was excluded by fluorescence in situ hybridization (FISH). Few cases of AML with MLL gene duplication have been reported, and they showed involvement of the M1, M2, and M4 subtypes. This case is the first reported case with an M6 phenotype, and highlights the importance of investigating for MLL gene mutations in all subtypes of AML, as they may carry prognostic significance.