Breast cancer is the most frequently diagnosed cancer in women worldwide and the second cause of cancer-related mortality. A total of 20-30% of patients with early-stage breast cancer develop recurrence within the first 5 years following diagnosis. Trastuzumab significantly improves overall survival and disease-free survival (DFS) in women with human epidermal growth factor receptor 2 (HER2)-positive early and locally advanced breast cancer. This study aimed to determine the factors that affect DFS following adjuvant transtuzumab therapy. A total of 62 patients treated with trastuzumab for early and locally advanced breast cancer were included in our study. Data, including pathology, treatment and treatment outcome, rate of recurrence and laboratory tests, were retrospectively collected. There was no significant association between DFS and age, menopausal status, disease stage and hormone receptor status. The median follow-up was 48.4 months. The median DFS of patients treated with adjuvant trastuzumab was 64.1 months. In addition, the median DFS was 44.3 vs. 66.8 months in patients with platelet-lymphocyte ratio (PLR) ≤200 vs. >200, respectively (log-rank test; P=0.001), and 70 vs. 45 months in patients with eosinophil count ≤70 vs. >70×103/mm3 (log-rank test; P=0.001). Our data revealed the prognostic relevance of a decrease in the peripheral blood eosinophil count and PLR value following trastuzumab therapy in breast cancer. PLR and eosinophil count measurements are cost-effective, readily available worldwide, non-invasive and safe. Combined with other markers, such as patient age, tumor stage and tumor histology, may be effectively used for patients with breast cancer.