The chromosomal abnormality der(9)t(1;9)(q11;q34) is a rare occurrence in patients with hematologic malignancies. As far as we know, only 3 cases of acute myeloid leukemia, 1 case of polycythemia vera, and 1 case of multiple myeloma with this derivative chromosome have been reported in the literature. Here we report the first case of der(9)t(1;9)(q11;q34) in a patient with chronic myelomonocytic leukemia (CMML). A 45-yr-old man was brought to our hospital for evaluation of pancytopenia and monocytosis. The patient's persistent monocytosis in peripheral blood and his bone marrow findings were consistent with the diagnosis of CMML. Chromosome study results repeatedly showed 46,XY,der(9)t(1;9)(q11;q34). In addition, the BCR/ABL fluorescent in situ hybridization (FISH) pattern of the interphase cells was interpreted as: "nuc ish(ABL, BCR) x 2[292/300]," consistent with the normal signal patterns found in 97% of the nuclei examined. For further evaluation, multi-color FISH (mFISH) analysis was performed and it showed the distinct unbalanced derivative chromosome der(9)t(1;9)(q11;q34) in 5 metaphase cells analyzed. Not only does this show an extraordinary type of trisomy 1q, but it reveals a rare recurrent case of der(9)t(1;9)(q11;q34) in patients with monocytic-lineage leukemia. Further studies are needed to evaluate the prognosis, survival, and treatment response of such patients with der(9)t(1;9)(q11;q34).