Background: IGF-1 has been shown to promote cancer growth by activation of PI3 kinase and downregulation of AMPK. IGFBP-1 shuttles IGF-1 across blood vessel membranes. Elevated IGFBP-1 blood levels may be indicative of tissue IGF-1 levels and has been associated with prostate cancer risk. We hypothesized that IGF-1 and IGFBP-1 blood levels might effect ADT efficacy. Methods: A retrospective analysis of IGF-1 and IGFBP-1 was undertaken in men commencing ADT for metastatic prostate cancer. Blood was drawn within one month of initiating ADT and stored. Proteins were measured using multiplex electrochemiluminescence assays. A cohort of 122 patients was compiled from a prospective trial (50 patients) and chart review (72 patients). A multivariate analysis (MVA) using a COX model was performed to identify associations between blood levels of the markers and time to the development of castration resistant prostate cancer (CRPC) and overall survival (OS). The covariates analyzed were age at time of ADT, ECOG score, race, baseline PSA, and extent of disease. Results: At time of initiating ADT the cohort had a median age of 67 years, PSA of 27.4 and 82% had ECOG 0. 91% were Caucasian. 62% had extensive metastatic disease. Median OS was 42.2 months. The median (med) baseline levels for IGF-1 was 75.6 ng/ml and for IGFBP-1 was 1844 pg/ml. As detailed in the table, patients with IGFBP-1 levels above the median had significantly shorter time to development of CRPC and OS on MVA. There was no association between IGF-1 levels and time to CRPC development or OS. Conclusions: Elevated levels of IGFBP-1 are associated with shorter duration of ADT efficacy in men with metastatic prostate cancer. This correlative data supports the ongoing SWOG trial assessing whether IGF signaling blockade improves ADT efficacy, as has been seen in preclinical models.