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Purpose: Discordance in human epidermal growth factor receptor 2 (HER2) status between primary tumor and metastases might have important implications for treatment response and therapy decisions. Here, we evaluate both the frequency of circulating tumor cells (CTCs) and the factors predicting HER2 discordance between primary tumor and CTCs as a potential surrogate for tumor biology and tumor heterogeneity in patients with metastatic breast cancer., Patients and Methods: The number of CTCs in 7.5 mL of peripheral blood and HER2 status were evaluated in 1,123 women with HER2-negative metastatic breast cancer. HER2 discordance was defined as the presence of at least one CTC with a strong immunocytochemical HER2 staining intensity. Factors predicting discordance in HER2 phenotype were assessed using multivariable logistic regression., Results: Overall, 711 (63.3%) of 1,123 screened patients were positive for CTCs (≥ one CTC). Discordance in HER2 phenotype between primary tumor and CTCs was observed in 134 patients (18.8%) and was significantly associated with histologic type (lobular v ductal; odds ratio [OR], 2.67; 95% CI, 1.63 to 4.39; P < .001), hormone receptor status (positive v negative; OR, 2.84; 95% CI, 1.15 to 7.02; P = .024), and CTC number (≥ five v one to four; OR, 7.64; 95% CI, 3.97 to 14.72; P < .001)., Conclusion: HER2 discordance between primary tumor and CTCs was observed in 18.8% of patients and was associated with histologic type, hormone receptor status of the primary tumor, and CTC number. The clinical utility of CTCs as liquid biopsy to assess tumor heterogeneity of metastatic disease and guide treatment decisions must be evaluated in prospective randomized trials.