Neoplasms have a striking tendency to metastasize or “home” to bone. Hematopoietic cells also home to bone during embryonic development, where evidence points to the chemokine stromal cell-derived factor-1 (SDF-1 or CXCL12; expressed by osteoblasts and endothelial cells) and its receptor (CXCR4) as key elements in these processes. We hypothesized that metastatic prostate carcinomas also use the SDF-1/CXCR4 pathway to localize to the bone. To test this, levels of CXCR4 expression were determined for several human prostate cancer cell lines by reverse transcription-PCR and Western blotting. Positive results were obtained for cell lines derived from malignancies that had spread to bone and marrow. Prostate cancer cells were also observed migrating across bone marrow endothelial cell monolayers in response to SDF-1. In in vitro adhesion assays, pretreatment of the prostate cancer cells with SDF-1 significantly increased their adhesion to osteosarcomas and endothelial cell lines in a dose-dependent manner. Invasion of the cancer cell lines through basement membranes was also supported by SDF-1 and inhibited by antibody to CXCR4. Collectively, these results suggest that prostate cancers and perhaps other neoplasms may use the SDF-1/CXCR4 pathway to spread to bone.