Endothelin-1 [ET-1] acts as a growth factor in various malignancies. Big endothelin-1 (big ET-1) is the precursor of ET-1. The aim of this study was to determine the importance of serum big ET-1 levels as a novel marker of disease in female patients with breast cancer of various clinical stages. The study consisted of 75 female patients with breast cancer who were diagnosed and treated at the Gazi University Department of Medical Oncology and 20 controls. The patients were classified as follows: group A [n=26], patients with newly diagnosed primary breast cancer but without metastasis; group B [n=33], patients with metastatic breast cancer who had undergone treatment for their diseases and in whom metastasis was detected during follow-up; group C [n=16], off-therapy patients whose cancer had been in remission for at least 5 years; and group D [n=20] healthy controls. Serum big ET-1 level were measured with an enzyme immunoassay kit. The median serum big ET-1 levels of the 75 patients with breast cancer [10.96 +/- 1.36 ng/ml] were statistically significantly higher than those of controls [8.97 +/- 1.55 ng/ml]. The median serum big ET-1 levels of the patients with primary breast cancer patients [group A] were statistically significantly different from those in the controls, the off-therapy patients and the patients with metastatic disease [11.56 +/- 0.78 ng/ml, 8.97 1.55 ng/ml, 9.76 +/- 1.52 ng/ml, and 10.83 +/- 1.18 ng/ml respectively, P=.001]. There was no statistically significant difference in the serum big ET-1 levels of patients in group A in terms of tumor stage, hormone receptor status or lymph node status. Serum big ET-1 levels were statistically significantly higher in patients with metastatic disease than in controls or off-therapy patients (P=.001). The serum big ET-1 levels of off-therapy patients whose disease was in remission were not statistically significantly different from those in controls (P>.05). Serum big ET-1 levels seemed to represent the activation of ET-1 in female patients with breast cancer. Serum big ET-1 levels can be an indicator of the breast cancer. Further studies are needed to demonstrate the prognostic importance of serum big ET-1 in patients with breast cancer. (C) 2008 Elsevier B.V. All rights reserved.