<p>Breast cancer is the most common malignancy among women worldwide, causing &gt;650,000 deaths annually. Despite advances in personalized therapies, many patients fail to obtain durable benefits from treatments tailored to their disease subtype. For instance, only some patients with PD-L1⁺ triple-negative breast cancer (TNBC) respond to chemotherapy plus immune checkpoint inhibitors. Moreover, while tumor-infiltrating lymphocyte levels correlate with improved survival in TNBC and HER2⁺ breast cancer, this is not true for HR⁺HER2^- disease. Thus, other immunobiological features of the breast cancer microenvironment may hold clinically relevant prognostic or predictive value, especially in HR⁺HER2^- tumors. Here, we investigated transcriptional signatures of immune cell infiltration across three public transcriptomic datasets from patients with breast cancer. The relative abundance and activation status of tumor-infiltrating mast cells were consistently associated with shifts in survival indicators amongst patients with HR⁺HER2^- breast cancer. Specifically, infiltration by resting (but not activated) mast cells was associated with prolonged survival, correlating inversely with tumor infiltration by immune cells and proliferation markers in cancer cells, but positively with stromal richness. While based on retrospective transcriptional analyses, these findings suggest that interactions between mast cells and non-malignant components of the breast cancer microenvironment, particularly fibroblasts, may shape disease progression and treatment sensitivity.</p><p></p>