De novo acute myeloid leukemia with trilineage myelodysplasia (AML/TMDS) is an uncommon form of leukemia characterized by a dyshematopoietic picture accompanying the acute leukemia, a poor response to induction chemotherapy, and a tendency to relapse with pure myelodysplastic syndrome. Cytogenetic information on this entity is scarce, although some cases have been reported to be associated with t(7;11)(p15;p15). A 41-year-old woman who had a history of radiotherapy for breast cancer presented with AML/TMDS and was found to have a unique t(11;12)(p15;q13) abnormality.