Background: Obesity has been shown to impact the prognosis of early BC; this effect was not consistently observed across the different biological subtypes, and is less clear when aggressive tumor phenotypes are considered. The aim of this study was to evaluate the influence of BMI (kg/m2) on the prognosis of women with high risk early BC enrolled into a phase III clinical trial of adjuvant CT. Methods: The relationship between BMI and Disease Free (DFS) or Overall Survival (OS) was assessed in 1066 early BC patients with rapidly proliferating tumors (Thymidine Labeling Index > 3% or G3 or Ki67 > 20%), randomized to receive adjuvant CT with or without anthracyclines (Epirubicin → CMF vs CMF → Epirubicin vs CMF). BMI was defined as follows: normal < 25 kg/m2, overweight 25-30 kg/m2, obese >30 kg/m2. DFS and OS were calculated by Kaplan-Meier estimation; multivariate Cox analysis was performed according to menopausal status, type of CT, hormonal, HER-2 and nodal status. Results: Information on BMI at baseline, was available on 959 women. Of these, 430/959 (44.8%) were normal, 331 (34.5%) were overweight and 198 (20.7%) were obese. Median age was 52 years (range 26 to 70); 48% was node positive, 62% was ER positive and 33% was HER-2 positive. At a median follow-up of 69 mos (range 1-119), 5-year DFS was 80% (95% CI 78-83) and 5-year OS was 94% (95% CI 90–94). 5-year DFS was 81% (95% CI 77-85), 82% (95% CI 77-86) and 76% (95% CI 70-83), in normal, overweight and obese women, respectively (p 0.6). 5-year OS was 92% (95% CI 89-95), 94% (95% CI 91-96) and 89% (95% CI 84-93), respectively (p 0.4). By multivariate analysis only ER, HER-2 and nodal status were significantly associated with differences in DFS and/or OS. Conclusions: BMI at baseline was not associated with the prognosis of early BC patients with rapidly proliferating tumors, receiving adjuvant CT. These results confirm those achieved in triple negative BC and suggest that neither dietary restriction or medical interventions aimed at reducing BMI and/or underlying insulin resistance nor specific anticancer strategies seem to be appropriate in this subgroup.