A 69-year-old man was admitted because of macrocytic anemia and peripheral monocytosis: hemoglobin 75 g/L and white blood cells 16.0x10(9) /L with 22% monocytes. Five years prior, he had received CHOP regimen and radiotherapy for diffuse large B-cell lymphoma. Bone marrow was hypercellular with trilineage dysplasia and 2.4% blasts. Chromosome analysis showed 46,XY,t(1;11)(p32;q23),del(5)(q13q35),+8,inv(9)(p11q13),-15,-21,+mar1. These findings indicated a diagnosis of therapy-related myelodysplastic/myeloproliferative neoplasms (t-MDS/MPN). Fluorescence in situ hybridization revealed that the breakpoint at 11q23 was centromeric to the MLL gene. Taken together with the previously reported cases, trilineage dysplasia and del(5q) without MLL rearrangement suggests that alkylating agents may have a crucial role in the pathogenesis of t-MDS/MPN, which is a rare but recognizable entity.