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The purpose of this study was to evaluate the effect of adjuvant treatment with tamoxifen on the CK-19 mRNA+ cells in patients with early-stage breast cancer.CK-19 mRNA+ cells were prospectively and longitudinally detected using a specific real-time PCR assay for CK-19 mRNA in 119 patients with estrogen and/or progesterone receptor-positive tumors during the period of tamoxifen administration.Twenty-two (18.5%) patients had detectable CK-19 mRNA+ cells after the completion of adjuvant chemotherapy and in 15 (68.2%) of them adjuvant tamoxifen could not eliminate these cells (persistently positive). In 68 (57.1%) patients, no CK-19 mRNA+ cells could be detected throughout the follow-up period (persistently negative). Seven (46.7%) of the 15 persistently positive and six (8.8%) of the 68 persistently negative patients developed disease recurrence (P=0.00026). Persistency of CK-19 mRNA+ cells was associated with a significantly lower median disease-free interval (P=0.0001) and overall survival (P=0.0005). Multivariate analysis revealed that the detection of CK-19 mRNA+ cells during the administration of tamoxifen was associated with an increased risk of relapse [hazard ratio (HR)=22.318, P=0.00006] and death (HR=13.954, P < 0.00001).The detection of CK-19 mRNA+ cells throughout the period of adjuvant tamoxifen treatment is an independent poor prognostic factor in patients with early breast cancer.