Purpose: Granulocyte macrophage colony-stimulating factor (GM-CSF) and thalidomide are active agents in prostate cancer. This study assessed the biological effects and safety of GM-CSF and thalidomide in patients with localized prostate cancer before radical prostatectomy. Experimental Design: Locally advanced prostate cancer patients undergoing radical prostatectomy were recruited for this study. Treatment consisted of two 28-day cycles of GM-CSF (250 u.g, s.c., thrice weekly) and thalidomide (200 mg, orally, daily) on days 1 to 28 of each cycle. Radical prostatectomy occurred within 7 to 10 days after completion of therapy. Pretreatment and posttreatment specimens were used to assess the expression of CD3, CD68, Ki-67, S100, PTEN, and CD31. Peripheral blood was examined for dendritic cells, regulatoryTcells, and cytokines. Results: Twenty-eight patients were enrolled. No pathologic responses (P0) were observed and no unexpected toxicities or surgical complications occurred. Eighty-one percent of patients had a prostate-specific antigen decline (mean ± SD decrease was 21.1 ± 15.4%; median, 18.0%). With a median follow-up of 32 months, five patients have experienced progression. Radical prostatectomy tumor tissue specimens showed significant CD3 and S100 overexpression when compared with pretreatment biopsies. No significant changes in tumor macrophage infiltration were observed. Increased number of serum dendritic cell, as well as high serum levels of interleukin-8, basic fibroblast growth factor, and vascular endothelial growth factor, was also observed. Conclusions: Neoadjuvant GM-CSF and thalidomide was safe and feasible and did not affect the perioperative morbidity of radical prostatectomy. Although no pathologic complete responses were observed, significant posttreatment tumorT-cell and dendritic cell infiltration was noted. No significant changes in serum cytokines, dendritic cells, or regulatoryTcells were induced. © 2008 American Association for Cancer Research.