Deregulation of the endothelin system, comprised of endothelin-1 (ET-1), its isoforms (ET-2 and ET-3) and their receptors (ET(A)R and ET(B)R), is under investigation in various types of human cancer. ET-1 has been suggested to participate in breast cancer development and progression, while Big ET-1, its biological precursor, has also been found elevated in breast cancer patients. In the present study, we investigated plasma ET-1 and Big ET-1 levels in patients with suspicious mammographic lesions, in order to assess their potential application as diagnostic biomarkers in the early estimation of breast disease. The study consisted of 94 patients (Group A to 30 patients with invasive ductal carcinoma: Group B, 30 with ductal carcinoma in situ; and group C, 34 with papilloma or ductal hyperplasia), who underwent an image-guided vacuum-assisted breast biopsy, and 30 healthy controls (group D). ET-1 and Big ET-1 plasma levels were measured with enzyme-linked immunosorbent assay. ET-1 levels did not exhibit significant differences between patients and healthy controls (Group A to 0.92 fmol/mL; Group B: 0.90 fmol/mL; Group C: 0.66 fmol/mL; and Group D: 0.86 fmol/mL). In contrast, Big ET-1 levels were significantly higher in patients with invasive or in situ carcinoma compared to healthy controls (Group A: 0.69 fmol/mL; Group B, 0.62 fmol/mL; and group D: 0.39 fmol/mL; p < 0.001 and p < 0.01). Plasma Big ET-1 may provide a useful tool for the early detection of invasive or noninvasive ductal breast cancer. The utilization of such a diagnostic tool would greatly assist in the modern management of breast cancer.