Background: Adjuvant breast cancer chemotherapy (ACT) improves relapse free (BCRFS) and overall survival (OS). Differences in terms of efficacy and toxicity could partly be explained by the significant interpatient variability in pharmacokinetics which cannot be captured by dosing according to body surface area. Consequently, tailored dosing was prospectively evaluated in the phase III PANTHER trial with 2017 patients. Methods: PANTHER is a multicenter, open-label, randomized phase III trial which compared tailored, dose dense epirubicin/cyclophosphamide (EC) and docetaxel (D) (group A) with standard interval 5-fluorouracil/E/C and D (group B), with identical duration of therapy in both arms. The primary endpoint was BCRFS. The primary efficacy analysis revealed an improved event free survival and favorable trends for BCRFS, OS and distant disease free survival compared with standard ACT. In this secondary analysis, we aimed to explore the concept of dose tailoring. Our two hypotheses regarding patients treated at group A were that BCRFS would not differ according to the cumulative administered dose; and that dose tailoring would lead to appropriate dosing and improved outcomes for obese patients. Results: Patients randomized in group A had similar BCRFS regardless of the cumulative epirubicin (p = 0.495) or docetaxel dose (p = 0.575). There were consistent, non-statistically significant trends in favor of patients receiving < 360 mg/m² epirubicin compared to 360-420 and > 420 mg/m². In addition, there were no differences in outcomes between patients receiving tailored ACT with a body mass index (BMI) of < 24 (n = 307), 24-28 (n = 296) and 28-40 (n = 312) (p = 0.384). Patients with a BMI of 28-40 had a non-significant trend for improved BCRFS, using BMI < 24 as reference (HR = 0.73, 95% CI 0.45-1.18) Conclusions: Dose tailoring could spare patients from unnecessary overdosing without compromising outcomes and may overcome the negative impact on prognosis conferred by obesity. Although exploratory, these results highlight the feasibility of tailored ACT, and underscore the need for further studies. An in-depth discussion will be presented at the ASCO meeting. Clinical trial information: NCT00798070