We describe an unusual case of chronic myelomonocytic leukemia with severe eosinophilia having t(5;12)(q31;p13) with t(1;7)(q10;p10). The eosinophilic proliferation was severe in peripheral blood and bone marrow, and they revealed marked dysplastic features. We performed fluorescence in situ hybridization (FISH) and immunohistochemistry to evaluate the clonality of eosinophils. The eosinophils were stained positively to platelet-derived growth factor receptor-β. By FISH using chromosome 1 satellite probe and chromosome 1q telomere probe, the eosinophils were proved to belong to the malignant clone.