The authors present two cases of patients with breast cancer with lymph node extension and who both had surgery. As a pancytopenia with hypercellular bone marrow was discovered at the same time in the first patient, she received no complementary treatment; 4 months later, she presented with an acute lymphocytic leukemia (ALL) for which a remission was easily induced, but she died of a pulmonary infection. The second patient received local radiotherapy (50 grays) and adjuvant chemotherapy (Alkeran for 26 months). Forty-seven months after the diagnosis of breast cancer and 16 months after the end of the treatment, an acute nonlymphoblastic leukemia (ANLL; M6) was diagnosed after 8 months of a preleukemic state. Treatment did not produce any results and death occurred on the 17th day. Cytogenetic studies on the bone marrow cells of both patients were performed. In the first patient in the ALL phase normal cells coexisted with a 47 chromosome clone, the extra chromosome being a D (+ 13?). In the second patient, several karyotype abnormalities were already present in the preleukemic state and also during the acute leukemic phase. No normal mitoses were found; hypodiploidy was present as well as major abnormalities such as markers, rings, and, among others, the systematic loss of a #5 and a #7. The first patient seems to have presented with a de novo ALL, associated with the malignant tumor; whereas, the second patient showed all the characteristics of an induced ANLL. The clinical, hematologic, and cytogenetic characteristics of these two patients are analyzed and compared to those of other cases in the literature.