Body size is associated with survival in women with breast cancer in complicated ways. Chemotherapy to treat breast cancer is typically dosed based on body size, often as a function of body surface area. Previously, there was concern that obese patients may receive doses of chemotherapy that may be too toxic given their higher body surface area, resulting in dose capping. This has resulted in the suggestion that obese women were receiving less-than-optimal doses for cancer treatment. Indeed, in 2012, the American Society of Clinical Oncology recommended that doses of chemotherapy not be capped. This project, enabled by availability of detailed infusion data, examines the impact of body size on chemotherapy dosing and subsequent outcomes. The project builds upon pilot work conducted in the Pathways Study cohort of 4,505 women with breast cancer, expands to other women with electronic data only, and includes over 4,000 women with breast cancer at Kaiser Permanente Washington. This project is led by Dr. Elizabeth Kantor at Memorial Sloan Kettering Cancer Center.