OBJECTIVES: Mixed results have been reported on the associations between insurance coverage and cancer treatment, probably due to different population involved. This study was conducted to investigate treatment patterns by insurance status in patients with breast cancer using a large national database. METHODS: The National Cancer Database (NCDB) was used to examine the initial treatment after diagnosis. Patients were classified into the early stage group (stage 0-2) and the advanced stage group (stage 3-4). Logistic regressions were used to estimate the Odds ratios (ORs, with confidence intervals [95% CIs]) of receiving surgery, hormone therapy, and chemotherapy by insurance status.  RESULTS: A total of 2,317,286 patients with breast cancer were retrieved from the database during the study period (2000-2011). Patients with private insurance, Medicare, Medicaid or no insurance accounted for 53.8%, 35.5%, 4.8%, and 2.2% respectively. Compared with the uninsured, patients with private insurance, Medicare, or Medicaid were more likely to receive surgery in both early stage group (OR 2.97, [2.83-3.11], OR 2.00, [1.90-2.09], OR 1.58, [1.49-1.67] respectively) and advanced stage group (OR 2.89, [2.78-3.01], OR 1.45, [1.39-1.51], OR 1.60, [1.53-1.68] respectively). In the early stage group, patients with private insurance and Medicare were more likely to undergo partial mastectomy than the uninsured (OR 1.40, [1.37-1.43], OR 1.36, [1.33-1.39]), while patients with Medicaid had similar utilization of partial mastectomy as  the uninsured (OR 1.02, [1.0-1.05]). In the advanced stage group, patients with private insurance, Medicare, or Medicaid were more likely to receive hormone therapy (OR 1.54, [1.48-1.60], OR 1.68, [1.61-1.74], OR 1.20, [1.15-1.26]) than the uninsured. Patients with private insurance and Medicaid were also more likely to receive chemotherapy (OR 1.67, [1.60-1.74], OR 1.39, [1.32-1.46]), while patients with Medicare were less likely to receive chemotherapy (OR 0.36, [0.34- 0.37]) than the uninsured. CONCLUSIONS: Insurance status is associated with different treatment patterns. It may have an impact on clinical management of patients with breast cancer.