Background: Third generation aromatase inhibitors have been widely used in postmenopausal women for the adjuvant treatment of hormone receptor positive breast cancer. As aromatase inhibitors work by inhibiting the conversion of androgens to oestrogens in adipose tissue the effectiveness of aromatase inhibitors might be dependent on body mass index (BMI). Methods: The ATAC study was a double-blind randomised clinical trial in which postmenopausal women with early breast cancer were randomly assigned to receive oral daily anastrozole (1mg) alone, tamoxifen (20mg) alone, or the combination in a double blind fashion. In this retrospective investigation, analyses were based on those women with hormone receptor positive breast cancers (oestrogen (ER) and/or progesterone (PgR) positive) for whom a baseline BMI measurement was available (anastrozole=2469; tamoxifen=2470). Here, we investigate the influence of BMI on time to recurrence (TTR) and time to distant recurrence (TTDR), and also the relative benefit of anastrozole versus tamoxifen according to baseline BMI, using a proportional hazard model. Results: After 100 months of median follow-up, obese women (BMI>30 kg/m 2 ) at baseline had a significantly higher breast cancer recurrence rate than lean women (BMI 2 ) (adjusted HR=1.27 (1.02-1.57), P trend = 0.003) (Table 1). Obese women also had 38% more distant recurrences compared to lean women (adjusted HR=1.38 (1.08-1.76), P trend Conclusions: These results confirm the poor prognosis of obese women with early breast cancer. Recurrence rates were lower in anastrozole than tamoxifen for all BMI quintiles and these results do not support an increased efficacy for anastrozole in obese women. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 1047.