Purpose: The purpose of this study was to investigate the incidence and prognostic relevance of tumor cell detection in granulocyte colony-stimulating factor-mobilized peripheral blood progenitor cell collections (PBPCCs) using cytokeratin (CK), maspin (MAS), and mammaglobin (MAM) genes as epithelial cell markers. The population on which the study was conducted was drawn from stage III breast cancer patients undergoing high-dose chemotherapy and autologous transplantation with PBPCCs. Experimental Design: One hundred and ninety-four patients were enrolled in the study and analyzed for tumor cell detection on the basis of 481 PBPCCs gathered before administration of chemotherapy. CK, MAS, and MAM gene expressions were investigated by means of the reverse transcription nested polymerase chain reaction, and those samples expressing CK were further hybridized with a radiolabeled internal probe to reduce false-positive results. Sensitivity and specificity were assessed on 37 controls (12 cell lines, 12 healthy donors, and 13 nonepithelial malignancies). Each of the known prognostic variables (age, stage, lymph node status, receptor status, c-ErbB2 status, and Ki67 status) was then analyzed (both individually and together with CK, MAS, and MAM expression on PBPCCs) in relation to patient overall survival (OS) and relapse-free survival (RFS). Results: After a 3-year follow-up, an estimated 83% (95% confidence interval, 77.1-88.8%) of the patients were alive and an estimated 67% (95% confidence interval, 60.1-74.6%) were free of relapse. One hundred and seventy-six of the 194 patients (91%) had contaminated PBPCCs evi-denced by at least one positive sample for any of the markers evaluated. The PBPCC frequency of CK, MAS, and MAM positivity (؉) was 71%, 36%, and 16%, respectively. MAM expression on PBPCC was associated with an increased risk of relapse (P ‫؍‬ 0.003), whereas CK and MAS expressions were not associated with changes in either RFS or OS. Conclusions: MAM gene expression on leukapheresis products of high-risk breast cancer patients is an indicator of poor prognosis. The method of evaluation is simple and reproducible and provides new tools for evaluating the role played by tumor cells in apheresis products and their potential in causing metastasis.