This report describes 2 patients who developed acute myelocytic leukemia (AML) type M2 and chronic myelomonocytic leukemia (CMML) of the FAB classification, respectively 2 months and 2 weeks after diagnosis of operable breast cancer. The patient with AML showed pancytopenia 2 months before the diagnosis of AML, had a normal karyotype, and showed a good response to chemotherapy. The patient with CMML had a normal karyotype, and she was treated with hydroxyurea and supportive therapy. The 2 patients had no previous exposure to irradiation or cytotoxic therapy. These cases show that breast cancer and either leukemia or myelodysplastic syndrome may be associated even without previous irradiation or combination chemotherapy.