The tumor-stroma ratio has previously been shown to be prognostic for patients with invasive breast cancer. We present a validation study to assess the prognostic significance in lymph node-negative, premenopausal patients from the perioperative chemotherapy trial (POP trial, 10854) conducted by the European Organization for Research and Treatment of Cancer. The POP trial assessed the efficacy of one course of perioperative chemotherapy (consisting of fluorouracil, doxorubicin, and cyclophosphamide). Hematoxylin and eosin (H&E) stained sections were retrieved from a subset of premenopausal, node-negative patients from this trial and were scored for the percentage of intra-tumoral stroma. The tumor-stroma ratio was associated with disease-free survival in univariate and multivariate analysis. Tumors with a high tumor-stroma ratio had an increased hazard of 1.853 for disease relapse (95 %CI 1.327–2.585, P  < 0.001) independent of other parameters. Combining other parameters with the tumor-stroma ratio improved risk stratification. For triple-negative tumors, the tumor-stroma ratio was associated with an increased hazard for disease relapse, independent of other parameters (HR 2.711, 95 %CI 1.111–6.614, P  = 0.028). The tumor-stroma ratio was also independently associated with locoregional recurrence even in breast cancer patients ≤40 years of age (HR 2.201, 95 %CI 1.038–4.669, P  = 0.040). This study validates the prognostic value of the tumor-stroma ratio. This parameter can be easily assessed on HE slides and can be implemented next to pathological staging reports to determine patient prognosis.