PURPOSE: Underweight women and those with obesity, defined as having a body mass index (BMI) ≥ 30 kg/m METHODS: Women age ≥ 15 years old diagnosed with BC between 2014 and 2019 were identified from the California Cancer Registry. BMI at diagnosis was classified as underweight (<
18.5 kg/m RESULTS: Of 159,248 patients: 2.2% were underweight, 34.6% normal weight, 30.5% overweight, 26.7% obesity class 1-2, and 6.0% obesity class 3. Compared to normal weight, patients who were underweight [Hazard Ratio (HR) 1.54, 95% Confidence Interval (CI) 1.51-1.57], obesity class 1-2 [HR 1.06, 1.05-1.07], and obesity class 3 [HR 1.14, 1.12-1.16] were more likely to be diagnosed with late-stage BC. In models stratified by age, patients ≥ 40 years who were underweight had worse BCSS, while patients ≥ 51 years with obesity class 1-2 had better BCSS. CONCLUSION: Patients with obesity class 1-2 were more likely to be diagnosed with a later stage, but had improved BCSS, supporting an "obesity paradox" in BC and suggesting that other measures are needed to better assess body composition, adipose distribution, and metabolic health of patients. Patients who were underweight had worse survival, suggesting this high-risk group may benefit from being assessed and treated for possible sarcopenia and malnourishment.