Background: We previously reported that circulating serum biomarkers of bone metabolism were significantly associated with overall survival (OS) in men with advanced prostate cancer, either in the hormone sensitive (Lara, et al. ASCO 2022) or castration resistant state (Lara, et al. JNCI 2016). In S1216, we showed that elevated bone biomarkers were significantly associated with an increased risk of death in HSPC regardless of bone metastases (mets). We also identified three risk groups (low, intermediate, and poor) with differential OS outcomes (median OS: 8.2, 5.1, and 2.1 years, respectively) based on combinations of bone biomarkers in these patients. Here we report the association of bone biomarkers with OS in men with HSPC and documented skeletal mets as part of a planned analysis of S1216. Methods: Bone resorption [C-telopeptide (CTx) and Pyridinoline (PYD)] and bone formation markers [C-terminal collagen propeptide (CICP) and bone alkaline phosphatase (BAP)] were assessed from patient sera. Patients were randomly divided into training (n=238) and validation (n=475) sets. In the training set, recursive partitioning that maximizes discrimination of OS was used to identify the dichotomous cut-point for each biomarker and for a combination of biomarker split points to define prognostic groups. In the validation set, Cox proportional hazards models were used to assess the impact of biomarkers on OS, adjusted for patient and tumor characteristics. Results: Of 1,279 men in S1216, 713 had both baseline bone mets and evaluable bone biomarkers. Patient characteristics were similar between the overall population and the subset with bone mets. Elevated levels of CICP, CTX, and PYD were strongly prognostic for OS, but not for BAP. Hazard ratios (95% CI) for OS adjusted for treatment arm and baseline clinical variables were: BAP – 1.31 (0.93, 1.84), p=0.12; CICP – 1.58 (1.09, 2.29), p<0.02; CTx – 1.55 (1.12, 2.15), p=0.008; and PYD – 1.66 (1.27, 2.217), p=0.0002. There was no evidence of interaction between elevated biomarkers and treatment (all p>0.2). Recursive partitioning algorithms identified four groups of patients with differential OS outcomes based on bone biomarkers, adjusted for baseline clinical variables, with median OS ranging from 2.3 years (highest risk group) to 7.5 years (lowest risk group). Conclusions: In this subset analysis of men with HSPC and bone mets in S1216, elevated serum markers of bone metabolism were significantly associated with worse OS. Bone biomarker levels alone and in combination with patient and tumor characteristics identify unique subsets of men with differential OS outcomes. Clinical trial information: NCT01809691.