ABSTRACT: Background:The prognostic nutritional index (PNI) is an indicator of nutritional immune status. Recently, the PNI has been found to be significantly associated with the clinical outcome of various solid tumors. Few patients with newly diagnosed breast cancer are in a state of malnutrition. In contrast, breast cancer is usually an overnutrition-related disease. This study aimed to explore the relationship of an excessively high PNI with sensitivity to neoadjuvant therapy and the prognosis of patients with locally advanced breast cancer. Methods:A total of 202 patients from two clinical trials, SHPD002 and SHPD003, were included. Binary logistic regression analysis was used to assess the association between the PNI and pathological complete response (pCR). Univariate and multivariate survival analyses were performed to assess the prognostic factors used to predict disease-free survival (DFS). Results:An excessively high PNI was more difficult to achieve pCR (OR =0.322; 95% CI, 0.132-0.788, P=0.013) and was associated with a worse DFS (log-rank P=0.013). The PNI was an independent prognostic factor for DFS in all patients (HR =3.027; 95% CI, 1.207-7.592, P=0.018), the premenopausal (HR =8.292; 95% CI, 1.670-41.17, P=0.010), clinical T3 and T4 (HR =3.405; 95% CI, 1.141-10.16, P=0.028), ER negative (HR =9.698; 95% CI, 1.205-78.07, P=0.033), HER2 negative (HR =3.765; 95% CI, 1.101-12.88, P=0.035) and pCR subgroups (HR =11.912; 95% CI, 1.326-107.0, P=0.027). Conclusions:An excessively high PNI was a risk factor for sensitivity to neoadjuvant therapy and prognosis of patients with locally advanced breast cancer.