41 patients with stage I and II breast cancer and 52 healthy controls were included into the study. HER2/ECD was determined before surgery and correlated with HER2/neu overexpression, Ki67, hormone receptor status and disease stage, and compared with value in healthy controls. Mean serum HER2/ECD concentration in patients was 8.62 ng/ml and 5.78 ng/ml in controls, and the difference was statistically significant (p = 0.000061). The best diagnostic cut-off value was 7.7 ng/ml, with 76.92% sensitivity and 72.92% specificity. Positive predictive value of the test was 69.77% and negative predictive value was 79.55%, with 74.71% of patients correctly classified. Serum HER2/ECD correlated with hormone receptors status, and no correlation with histological overexpression has been observed. CONCLUSION. Serum HER2/ECD concentration of ≥7.7 ng/ml has possible diagnostic value in stage I and II breast cancer. It should not be used as a determinant of HER2 positivity. Prognostic significance of HER2/ECD in early breast cancer, its correlation with hormone receptor status, and interconnection between hormone receptors and HER2 receptor signaling should be further analyzed, since it may have therapeutic implications.