To investigate the source of serum exosomal HOTAIR, to uncover the diagnostic and prognostic values of serum exosomal HOTAIR, and to discern the expression of serum exosomal HOTAIR between neoadjuvant chemotherapy and response to tamoxifen therapy. Samples were collected from the Third Affiliated Hospital of Kunming Medical University, Tumor Hospital of Yunnan. Exosomes were isolated from serum, cell culture medium and tumor tissues. We used transmission electron microscopy and western immunoblotting assay to characterize exosomes, and real-time PCR (qPCR) to assess HOTAIR expression. Neoadjuvant chemotherapy and tamoxifen therapy were carried out according to established guidelines. Breast cancer patients expressed higher serum exosomal HOTAIR than did healthy individuals (P\0.001). Serum exosomal HOTAIR levels 3 months after surgery were markedly decreased compared with levels before surgery (P\0.001), and the expression level of exosomal HOTAIR in cell culture medium increased with time in both breast cancer cell lines (72 h greater than 48 h greater than 24 h, 48 h vs 24 h [ [P less than 0.05]; 72 h vs 24 h [P less than 0.01]. Expression of serum exosomal HOTAIR in nude mice was notably greater than in the mock control group (P less than 0.001). The results of the ROC analysis revealed an AUC for serum exosomal HOTAIR of 0.9178 with a 95% CI of 0.8407-1.017 (P less than 0.01). The AUC for the CA15-3 cell line was 0.7378 (95% CI, 0.5585-0.9170; P = 0.03). High expression of exosomal HOTAIR led to a worse disease-free survival (P = 0.0481) and overall survival (P = 0.0463). In the high-expression chemotherapy group, six patients achieved a partial response (PR) and eight demonstrated stable disease (SD) and nine patients achieved PR and two SD in the low-expression group (P = 0.048). In the low-expression tamoxifen group, one patient had a recurrence of breast cancer and another 10 patients exhibited no recurrence, while six showed recurrence, and seven had none in the highexpression group (P = 0.035). We isolated exosomes successfully, and demonstrated that serum exosomal HOTAIR originated from primary breast cancer tissue. We conclude that serum exosomal HOTAIR exhibits the potential to be a diagnostic and prognostic biomarker. High expression of serum exosomal HOTAIR was also correlated with poor neoadjuvant chemotherapy and response to tamoxifen therapy.