Background: Increased acute myeloid leukemia (AML) incidence has been identified post chemotherapy and radiation treatment for primary breast cancer (BC). Risk has not been evaluated in large populations or included myelodysplastic syndrome (MDS). Methods: We used 2001-2009 Surveillance, Epidemiology and End Results (SEER) database records to identify a cohort of first primary stage I-III BC patients. We identified subsequent MDS/AML diagnoses in the BC cohort, using SEER to query appropriate ICD-O-3 codes. We compared observed MDS/AML rates in the BC cohort to expected rates, estimated for first primary MDS/AML in the entire population, and calculated observed/expected rate ratios with 95% confidence intervals (CI) with age adjustment. Due to SEER data limitations, disease stage was used as a proxy for likelihood of radiation and chemotherapy treatment. Results: BC case distribution by stage was 51% stage I, 39% stage II and 11% stage III. Age distribution was 26% 20-49, 38% 50-64 and 36% 65+ years. Out of 306,691 BC cases, 470 had a subsequent diagnosis of MDS or AML (.15%) with 19% among women age 20-49 years, 36% women age 50-64 and 45% among women age 65+ years. Age adjusted number of expected myeloid leukemia cases is 171. Follow up time to myeloid leukemia diagnosis was on average 2.92 years post BC diagnosis (range .25-8.75 years). Myeloid leukemia cases were 56% AML and 44% MDS. We found an overall increased risk of MDS/AML among all age (20-65+)/all stage (I-III) breast cancer cases compared to the general population (RR=2.75, 95% CI 2.51, 3.00). Risk increased with increasing BC stage: stage I (RR=1.87, 95% CI 1.61, 2.16), stage II (RR=3.57, 95% CI 3.12, 4.06), and stage III (RR=5.66, 95% CI 4.45, 7.07) and decreased with increasing age with the highest risk among women age 20-49 (RR=10.60, 95% CI 8.57, 12.92) (age 50-64 (RR=5.60, 95% CI 4.79, 6.49), age 65+ (RR=1.81, 95% CI 1.57, 2.06)). Conclusions: Patients with stage I-III breast cancer have a significant high risk of MDS/AML post BC diagnosis increasing with higher stage cancer, most likely the result of radiation and/or chemotherapy treatment. Younger age women, 20-49 years, appear to be the most susceptible to this outcome.