Up to 60% of patients with clinically localized prostate cancer will relapse despite potentially curative local treatment. Current staging tests have been limited in adequately identifying individual patients who are at a high risk for future relapse. Detection of bone marrow micrometastases may identify individuals destined to develop clinically detectable systemic metastases. Although immunohistochemistry and molecular approaches are being investigated, the most ideal test(s1 are yet to be determined. In this report we describe methods for specific detection and isolation of prostate cancer micrometastases by multi-parameter rare event flow cytometric analysis. A model was developed and validated using three human prostate cancer cell lines, healthy donor marrow, dual marker labeling for cytokeratin (epithelial-specific marker) and CD45 (bone marrow-specific marker). The detection sensitivity of this model was at the level of one prostate cancer cell in 100,000 nucleated bone marrow cells. As a part of an ongoing clinical study, bone marrow aspirates from 15 patients with newly diagnosed prostate cancer have been analyzed. Six patients were found to have cytokeratin positive/CD45 negative cells in their bone marrow aspirates. We conclude that flow cytometric rare event analysis provides a sensitive and specific assay for detection of bone marrow micrometastases in patients with clinically localized prostate cancer. o 1996 Wiley-Liss, tnc.