Background: BMI is associated with an increased risk of BC in postmenopausal women. Moreover, BC overweight women have poorer prognosis compared to women with normal weight. Data from ATAC (the anastrozole, tamoxifen alone or in combination study) trial reported that obesity reduce the efficacy of the aromatase inhibitor (AI) anastrozole in early BC. One possible explanation is that higher estrogens levels resulting from high BMI may lead to lower efficacy of the AI treatment. No clear data are available about the effects of BMI on estrogen circulating levels during AI treatment, and if the increase of AI dosage may result in more complete estrogen suppression in overweight women. Methods: We evaluated the correlation between BMI and plasma concentration of ES in postmenopausal women with early BC during adjuvant treatment with L (2.5 mg/die). Patients were participating in two prospective Italian clinical trials (GIM4 and GIM5) evaluating L treatments after adjuvant tamoxifen. Plasma samples were obtained after at least six weeks of L therapy. After a non-chromatographic cleaning procedure of samples, plasma ES levels were evaluated by RIA. Ln values for plasma ES were used for statistics. Geometric means and 95% confidence interval (CI) were used as summary measures. Results: ES plasma concentration and BMI were evaluated in 370 women. Median treatment duration was 49wks (range 6-201wks). Median age of patients was 60 yrs (range 34-84 yrs). Table shows data regarding ES concentrations and BMI of patients. Conclusions: Circulating ES levels during L (2.5 mg/die) treatment were super imposable in all BMI groups. Our data indicate that whether BMI influences AI efficacy, this is not due to an insufficient suppression of aromatase activity in women with higher BMI. Increasing the dose of AI to further reduce ES levels in high BMI patients is a questionable way to increase AI efficacy.