During apoptosis, a number of intracellular proteins are cleaved by caspases. The intermediate filament protein cytokeratin 18 (CK18) is cleaved at Asp238 and Asp396. A monoclonal antibody, M30, specifically recognises a fragment of CK18 cleaved at Asp396 (M30-antigen). We used an enzyme-linked immunosorbent assay (ELISA) to measure M30-antigen levels in the sera of 82 healthy subjects and 201 patients with breast cancer. Patients with primary cancer had higher M30-antigen levels than healthy subjects (P=0.0001). Patients with recurrent cancer showed higher M30-antigen levels than healthy controls and patients with primary cancer (P<0.0001 and P=0.008, respectively). In patients with primary cancer, M30-antigen levels were higher in the oestrogen receptor (ER)-negative subgroup than the ER-positive subgroup. In patients with recurrent cancer, M30-antigen levels correlated with the number of involved organs and performance status (P=0.041 and P=0.014, respectively). There was no association between serum M30-antigen levels and patient prognosis. We conclude that the levels of circulating M30-antigen are increased in patients with breast cancer.