Recent studies show that AC133-a hematopoietic stem cell antigen, when coex-pressed with endothelial markers, identifies a population of endothelial precursor cells (EPCs) in peripheral blood that function in tumor vasculogenesis in animals. Little is known about whether EPCs contribute to human tumor vasculogenesis. We attempted to determine if, through increased peripheral expression of AC133 or endothelial markers previously associated with EPCs,VEGFR-2 and Tie-2, we could detect an EPC response in the blood of patients with breast carcinoma. Thirty patients were segregated based on their breast biopsy histology into infiltrating carcinoma, DCIS and control groups. Using Real Time PCR, we measured the expression of the aforementioned markers in reverse transcribed RNA extracts from preoperative peripheral blood specimens. The cancer patients had significantly elevated Tie-2 expression with the highest levels associated with infiltrating carcinoma. Our data suggest increased circulating EPC markers in tumor patients, but further study of this cell population is needed to better define its role in tumor vasculogenesis.