BACKGROUND: Obesity is a risk factor for breast cancer (BC) development, recurrence, and death. In view of this, we aimed to investigate the clinical value of obesity in BC patients treated with anti-HER2 therapies in the NeoALTTO trial, which randomized 455 patients to neo-adjuvant lapatinib, trastuzumab, or their combination plus paclitaxel. METHODS: Patients were classified according to their basal body mass index (BMI) into underweight (textless 18.5 kg/m(2)), normal (≥ 18.5; textless 25 kg/m(2)), overweight (≥ 25; textless 30 kg/m(2)), and obese (≥ 30 kg/m(2)) WHO categories. Univariate and multivariate logistic regression analyses were performed using BMI as a categorical variable. Pathological complete response (pCR) and event-free survival (EFS) were the NeoALTTO primary and secondary outcomes, respectively. RESULTS: Among 454 patients analyzed, 14 (3, 220 (48, 137 (30, and 83 (18 were classified as underweight, normal weight, overweight, and obese, respectively; 231 (51 and 223 (49 had hormone receptor (HR)-positive and HR-negative primary tumors; 160 (35 achieved pCR. In the overall patient population, no association was found between BMI groups and pCR, as we reported pCR rates of 57.1 35 30.7 and 39.8 normal weight, overweight, and obese cases, respectively. In contrast, in HR-positive tumors, overweight or obesity was generally associated with decreased likelihood of achieving a pCR independently of other clinical variables, including planned surgery, nodal status, and tumor size (odds ratio [OR] = 0.55, 95.30-1.01, as compared to normal or underweight; p = 0.053); notably, no differential effect of BMI with respect to pCR was observed in HR-negative cases (odds ratio [OR] = 1.30, 95.76-2.23, as compared to normal or underweight; p = 0.331), resulting in a statistically significant interaction between BMI and HR status (p = 0.036). There was no association between BMI and EFS neither in the overall nor in the HR-positive population, but this analysis was under-powered. CONCLUSIONS: NeoALTTO patients overweight or obese at baseline and with HR-positive primary BC appeared less likely to achieve pCR after neo-adjuvant anti-HER2 therapies. This finding paves the way to future research in targeting the interplay between HER2/HR signaling and metabolism.