The association between obesity and prognosis in early breast cancer (EBC) is unclear, especially when aggressive phenotypes are considered. We evaluated the influence of BMI on the prognosis of women with high-risk EBC enrolled in a phase III trial of adjuvant chemotherapy (CT). The association was assessed in 1066 patients with rapidly proliferating tumors, randomized to receive adjuvant CT with or without anthracyclines. Disease-free survival (DFS) and overall survival (OS) were calculated by Kaplan–Meier; multivariate analysis was performed according to age, tumor size, nodal, estrogen receptor (ER), and HER2 status and type of CT. Information on BMI was available for 959 women. Of these, 529 (55.2 %) were overweight or obese. Median age was 52 years. A total of 457 (47.7 %) patients had nodal involvement. Centralized pathology was performed in 850 cases: 522 (61.4 %) were ER positive, and 194 (22.8 %) were HER-2 positive. At a median follow-up of 103 months (range 1–188), 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 = 0.44). Five-year OS was 92 % (95 % CI 89–95), 94 % (95 % CI 91–96), and 89 % (95 % CI 84–93), respectively (p = 0.60). BMI was not associated by multivariate analysis with differences in DFS or OS. Higher BMI had no influence on prognosis in high-risk EBC patients treated with CT. These data are consistent with prior observations and suggest that in aggressive biological subtypes, the impact of host factors on patient prognosis is minor.