Mortality among patients with breast cancer (BC) is mainly caused by metastasis. We determined the circulating tumor burden in BC patients by semiquantitative reverse transcription-polymerase chain reaction using carcinoembryonic antigen (CEA) and cytokeratin 19 (CK19) mRNAs as molecular markers. We distinguished the mRNA levels in circulation between BC patients and healthy controls with reference to a BC-derived cell line, SK-BR-3. We prospectively analyzed peripheral blood samples from 33 BC patients and 26 healthy controls. We found CEA mRNA in 97% of patients and 92% of normal controls, and CK19 mRNA in 72% of patients and 19% of controls. CEA and CK19 mRNAs in normal peripheral blood were most likely derived from illegitimate transcription. In 10 patients, of whom 9 (90%) developed systemic metastases, the upper limit of CK19 mRNA of normal controls was exceeded. As compared with normal controls, significantly elevated CK19 mRNA levels in the patients appeared to originate from circulating malignant BC cells (P<0.0001). It was clinically significant that the mean CK19 mRNA level increased with advancing disease stage. Of prognostic value, we report for the first time that BC patients with CK19 mRNA elevation had notably shorter (approximately 3-year reduction) overall survival than patients with normal CK19 mRNA levels (P=0.045). Quantification of CK19 mRNA may prove useful for cancer staging, disease monitoring and prognostic assessment among BC patients.