M30 and M65 are relatively new assays that detect different circulating forms of the epithelial cell structural protein cytokeratin18. The objective of this study was to determine the clinical significance of the serum levels of M30 and M65 in patients with breast cancer. A total of 80 patients with a pathologically confirmed diagnosis of breast cancer were enrolled into the study. Serum M30 and M65 concentrations were determined by the solid-phase sandwich ELISA method. Serum samples were obtained on first admission before any type of treatment. The median age at diagnosis was 52 years, range 30 to 81 years. The baseline serum M30 and M65 levels in patients with metastatic disease were significantly higher than those in the non-metastatic patients (P = 0.017 and P = 0.003, respectively). Moreover, serum M65 level was also elevated in patients with large tumor size (P = 0.02). No correlation was found between these serum assay levels and response to chemotherapy (P > 0.05). However, the significant relationship was found between the serum levels of M30 and M65 (r(s) = 0.96, P < 0.001). Neither serum M30 nor serum M65 had significantly effect on survival (P = 0.50, and P = 0.52, respectively). In conclusion, although both serum M30 and M65 levels are elevated in metastatic disease, no predictive and prognostic roles on survival were found in patients with breast cancer. (C) 2014 Elsevier Masson SAS. All rights reserved.