The MLL gene at chromosome band 11q23 is frequently rearranged and fused to partner genes in acute leukemias. Previously, the MSF gene, also called AF17q25, has been cloned as a fusion partner of the MLL gene in therapy-related or infant acute myelogenous leukemias with t(11;17)(q23;q25). MSF belongs to the septin family of proteins, which includes other MLL fusion partners, hCDCrel1 and Septin 6, and has also been implicated in the pathogenesis of human ovarian tumor and murine T-cell lymphoma. We describe here a 64-year-old man with de novo acute myelomonocytic leukemia (French-American-British subtype M4) with t(11;17)(q23;q25). His leukemia was successfully induced into a first remission, which, however, lasted only briefly. A second remission was never attained, and the patient died of sepsis 16 months after the diagnosis of leukemia. Examination of his leukemic cells at diagnosis revealed an MLL gene rearrangement, by Southern blotting, and an MLL-MSF fusion transcript, by the reverse transcriptase polymerase chain reaction (RT-PCR) method. Sequence analysis of the RT-PCR product further revealed that MLL exon 5 was fused in-frame to MSF exon 3. Further clinical and molecular analyses of acute leukemias with the MLL-MSF transcript may shed more light on the clinical characteristics and molecular mechanisms of the MLL-septin type leukemias.