The eosinophilic variant of acute myelomonocytic leukemia (AMML) is highly associated with characteristic abnormalities of chromosome 16. Leukemias developing secondary to chemotherapy frequently show abnormalities of chromosomes 5 and 7. This report describes a secondary AMML, eosinophilic variant (M4Eo), developing in a patient with coexistent mycosis fungoides who had received chemotherapy, including an alkylating agent, and showing characteristic abnormalities of both chromosomes 7 and 16. This is the first reported case of the M4Eo arising secondarily in a patient treated for another hematopoietic malignancy, and the second case of treatment-induced M4Eo to show coexistent abnormalities of chromosomes 7 and 16. Additional cytochemical, ultrastructural, and cell surface marker findings are described supporting malignant transformation of the eosinophilic cell line.