Cancer-associated microRNAs have been stably detected in blood. The objective of this study was to identify a panel of circulating microRNAs with the potential to serve as biomarkers for estrogen receptor-positive (ER+)/human epidermal growth factor receptor 2 (HER2)- breast cancer. We used microarray-based expression profiling to compare the levels of circulating microRNAs in blood samples from 11 ER+/HER2- advanced breast cancer patients plus 5 age-matched controls. MicroRNA levels were validated by reverse transcription quantitative polymerase chain reaction in 40 control subjects, 187 early breast cancer patients, and 45 metastatic breast cancer patients. Then, we assessed the association between the levels of microRNA and clinical outcomes of ER+/HER2- metastatic breast cancer. Initially, we found that miR-1280, miR-1260, and miR-720 were up-regulated in blood from breast cancer patients (P < 0.05). In validation, miR-1280 levels significantly increased in breast cancer patients and reflected tumor status (control<<early cancer<metastatic cancer). Among 37 metastatic breast cancer patients, miR-1280 levels significantly decreased after treatment in patients who responded to systemic treatment (P < 0.001). We confirmed that miR-1280 was not a classic microRNA, but a tRNA(Leu)-derived fragment. These findings suggest that a circulating tRNA-derived microRNA, miR-1280, is differently expressed in breast cancer patients and may serve as a biomarker for ER-positive breast cancer.