Background: The objective of this study was to identify prognostic factors affecting the recurrence-free survival (RFS) in patients who received a 52-week trastuzumab therapy for HER2-positive early stage breast cancer (EBC). Patients and Methods: The medical records of all patients with EBC from 10 centers were analyzed. Pathologic and clinical tumor characteristics were evaluated in 424 female patients who received 52 weeks of adjuvant trastuzumab for HER2-positive EBC. Survival was estimated using the Kaplan-Meier method. Univariate analyses of RFS were performed with the log-rank test. Independent prognostic and predictive factors affecting RFS were assessed by Cox regression analysis. Results: Median follow-up time was 33.1 months (range 9.2-75.9 months). 3-year RFS and overall survival were 87 and 97%, respectively. In multivariate analysis, patients aged 70 years or over (p = 0.017, relative risk (RR) 2.7, 95% confidence interval (CI) 1.19-6.13), patients with > 9 positive lymph nodes (p = 0.001, RR 2.52, 95% CI 1.42-4.46), and those with progesterone receptor-negative tumors (p = 0.006, RR 2.33, 95% CI 1.27-4.27) had worse RFS. Conclusion: In spite of a 52-week adjuvant trastuzumab treatment, classic poor prognostic factors for invasive EBC remained as such in patients with HER2-positive EBC.