Abstract BACKGROUND: Obesity has risen to epidemic proportions and is associated with worse breast cancer (BC) prognosis in most studies. However, the effects of obesity according to adjuvant therapy choice are largely unknown. To address this issue, we examined the relationship between body mass index (BMI), adjuvant therapy, and survival outcomes in a large cohort of early-stage BC patients. METHODS: We retrospectively studied patients from the Baylor Breast Center Tumor Bank treated from 1970–1995. Patients were divided into 3 BMI classes: normal/underweight (N, BMI<25), overweight (Ov, BMI 25–30), obese (Ob, BMI≥30); and 4 treatment groups: no adjuvant therapy, chemotherapy (mainly CMF), endocrine therapy (mainly tamoxifen), both chemo- and endocrine therapy. Time-to-recurrence (TTR), disease-free survival (DFS) and overall survival (OS) were estimated by the Kaplan-Meier method and compared among groups via the log-rank test. Multivariate analysis was conducted via Cox proportional hazards models. RESULTS: There were 4,368 patients. Median age was 58. 74% were postmenopausal. 72% had stage I-II disease, 28% stage III. 76% were estrogen receptor (ER)-positive, 24% ER-negative. Patients distributed into BMI classes as follows: N 48%, Ov 30%, Ob 22%. Higher BMI was associated with postmenopausal status and increasing age, tumor size, positive lymph nodes, and stage, as well as a higher likelihood of receiving treatment. Median follow-up was 5 years. Kaplan-Meier analysis showed that TTR was significantly shorter in the Ov and Ob groups as compared to the N group (p=0.019), due to distant (p=0.001) rather than local (p=0.970) recurrences. DFS was also significantly worse in the Ov and Ob groups (p=0.002), as was OS (p=0.001). The Table shows the hazard ratios for the various survival outcomes after adjustment for age, tumor size, nodal status, and treatment groups. For all patients, TTR, DFS, and OS were significantly worse in the Ob vs. N groups. TTR and DFS were significantly worse in the chemo treated Ob vs. N groups. DFS and OS were significantly better in the endo treated Ov vs. N groups. DISCUSSION: In this large cohort of BC patients, survival outcomes (TTR, DFS, OS) were significantly worse in the obese group. This remained true after adjustment for multiple factors. Obesity was associated with worse survival outcomes in the chemo treated (CMF) group. Overweight was associated with better survival outcomes in the endo treated (tamoxifen) group. These results confirm and extend the results of previous studies. Further studies to discover the reasons for these differences in outcomes are underway. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-08-04.