We studied MLL rearrangements in five patients with myeloid hematologic malignancies with trisomy 11. Two had acute monocytic leukemia (AMoL), one had chronic myelomonocytic leukemia, one had refractory anemia, and the other had juvenile chronic myelogenous leukemia. Only one patient, a 15-year-old boy with AMoL and simple trisomy 11, showed rearrangement of MLL. He did not respond to chemotherapy, and successfully underwent bone marrow transplantation, but suffered a relapse 22 months later. Reverse transcription-polymerase chain reaction (RT-PCR) and sequencing analyses of bone marrow cells revealed a tandem duplication of MLL, and his relapse was predictable by sequential RT-PCR studies before it was clinically evident. Of 16 acute myeloid leukemia patients with trisomy 11 and rearrangement of MLL reported, our patient was the youngest in age and the only one with AMoL.