Background: Although ER-positive, HER2-negative early-stage breast cancer is considered to have a relatively good prognosis, it is known to occur long-term recurrence for more than 10 years. ¹⁸F-FDG PET/CT has been reported to be useful in predicting short-term recurrence of breast cancer (2-5 years), but its correlation with long-term prognosis over ten years in ER-positive HER2-negative breast cancer is not clear. In the present study, we examined whether tumor FDG accumulation of pre-treatment PET/CT can be a predictor of long-term overall survival (OS) in ER-positive, HER2-negative early-stage breast cancer patients. Methods: Of 639 primary breast cancer patients treated at our institution between January 2007 and May 2010, 332 patients with stage 2 or less and ER-positive HER2-negative tumors who were able to confirm whether or not they had recurred at ten years postoperatively were included in the study. The impact of patients' characteristics, tumor background, and tumor FDG accumulation of pretreatment PET/CT image on OS was investigated using Cox regression analysis. P < 0.05 was considered to be a statistically significant difference. Results: There were 33 (9.9%) recurrences and 13 (3.9%) deaths during a median observation period of 121 months. Univariate analysis showed that tumor SUVmax / FDG accumulation in lymph nodes / stage / N category / diameter of invasion / nuclear grade were candidate predictors of OS. Multivariate analysis revealed that SUVmax (Hazzard ratio 1.36 [95% CI 1.13-1.63], P < 0.001) was the only predictor of OS. Conclusions: In patients with ER-positive HER2-negative early-stage breast cancer, tumor SUVmax on pretreatment PET/CT was the only independent predictor of OS.