The tumor-associated stroma is an essential compartment in breast cancer, and collagen fiber organization in the stroma has been reported to be correlated with prognosis. In this study, we sought to evaluate collagen fiber characteristics in relation to pathological complete response (pCR) after neoadjuvant chemotherapy (NAC) in breast cancer patients.A total of 388 breast cancer patients receiving NAC were enrolled. The stroma type was manually assessed on pretreatment hematoxylin and eosin (HE)-stained slides, and the collagen fiber features were quantified by a computer tool. The relationship between syndecan-1 expression and collagen fibers and its correlation with treatment efficacy were detected by immunohistochemistry.The pCR rate of patients with collagen-dominant stroma was lower than that of patients with lymphocyte-dominant stroma (19.6% vs. 40.0%, p = 0.001). Patients who achieved pCR had straighter and less dense fibers in pretreatment biopsied tissue than non-pCR patients (p = 0.031, p = 0.044). Additionally, the pCR group had greater syndecans-1 expression on the tumor epithelium than the non-pCR group (p < 0.001), while there was no statistically significant difference in the stroma (p = 0.333). Collagen fiber density was the only factor associated with pCR after correction for other clinicopathological variables in triple-negative breast cancer (TNBC) patients (OR 0.466, 95% CI 0.227-0.956, p = 0.037); patients with lower fiber density had a greater pCR rate (37.5% vs. 12.5%, p = 0.021).Collagen fiber density was associated with pCR in patients with breast cancer, and it could be a potential candidate for discriminating between responders and nonresponders for TNBC patients receiving NAC.© 2024 Wiley Periodicals LLC.