Résumé : Purpose: Inferior overall response rate with abemaciclib plus endocrine therapy was observed in patients with hormone receptor-positive/HER2-negative advanced breast cancer (BC) and BMI ≥ 25. We assessed the impact of baseline BMI on KI67% changes, achievement of complete cell cycle arrest (CCCA), clinical, and radiological responses in patients included in the NEOMONARCH trial. Methods: Exploratory post hoc analysis of the NEOMONARCH trial was performed. Patients were classified according to baseline BMI into underweight/normal weight (BMI < 25 kg/m2) and overweight/obese (BMI ≥ 25 kg/m2). Results: 222 patients (84.4%) had baseline BMI information available. In the overall cohort, mean Ki67% changes at 2 weeks were similar between the two BMI groups: − 19 (IQR − 27.8 to − 10.4) for patients with BMI < 25 and − 17.2 (IQR − 26.8 to − 11) for patients with BMI ≥ 25 (p = 0.760). There was no statistical difference in patients achieving CCCA after 2 weeks of treatment according to BMI (p = 0.096). Mean Ki67% reduction at 2 weeks was significantly higher for patients receiving abemaciclib plus anastrozole when compared to either anastrozole or abemaciclib alone, regardless of BMI. At the end of treatment, there was no significant difference regarding radiological (p = 0.366) or clinical response (p = 0.261). Conclusion: BMI categorized by the threshold of 25 did not significantly impact KI67% changes or clinical and radiological response. Although limited by the small sample size, these results are reassuring that the combination of abemaciclib plus anastrazole appears to be active in the early setting regardless of baseline BMI. Trial registration: ClinicalTrials.gov identifier: NCT02441946.