Immunohistochemical studies proved that the presence of breast cancer (BrCa) is accompanied by elevated levels of epithelial membrane antigen (EMA) and decreased levels of cytokeratin-1 (CK1). We, therefore, hypothesize that the serum EMA/CK1 ratio may serve as a promising biomarker for early diagnosis of breast cancer. The circulating levels of EMA and CK1 were determined by Western blot and enzyme-linked immunosorbent assay (ELISA) in sera from 102 women with BrCa and 90 women as controls (40 with benign breast disease and 50 healthy). EMA at 130 kDa and CK1 at 67 kDa were identified, purified, and quantified in sera of BrCa patients using ELISA. EMA/CK1 ratio values were found to discriminate BrCa patients from controls (P < 0.0001) with high diagnostic ability (area under the curve [AUC] = 0.901, sensitivity = 82, specificity = 76). The sensitivity and specificity for early-stage (≤ T2) BrCa were 72 and 76%, respectively. The ratio values of patients with late-stage (>T2) tumors were significantly higher than those of patients with early-stage (≤ T2) tumors. Moreover, higher grades (grades 2-3) were associated with higher values than grade 1 tumors. AUC values in different BrCa patients who had early stage, low grade, or size ≤ 2 cm were 0.855, 0.762, and 0.839, respectively. AUC values of patients with positive lymph node or positive distant metastasis were 0.907 and 0.913, respectively. We show for the first time the impact of serum EMA and CK1 ratio in BrCa detection. Differential EMA/CK1 values may serve as a diagnostic marker in early-stage breast cancer patients.