Background: Enzalutamide (ENZ), an androgen receptor antagonist therapy, was approved for patients (pts) with mCRPC. However, in standard of care for mCRPC pts, change in prostate specific antigen (PSA) is not accepted as an efficacy response measurement and the radiological change is inadequately measured in an interpreter-dependent subjective analysis of bone scan. Therefore, an objective efficacy response biomarker is warranted. In this registry study, we evaluated BSI as a quantitative analysis of bone scintigraphy, to access response in mCRPC pts being treated with ENZ. Methods: Pts with mCRPC, at Skåne University Hospital (SUH), Sweden, who initiated treatment with ENZ after failing chemotherapy were eligible for the study. Primary objective was to associate the change in BSI and PSA, after 12 weeks (wks) of treatment with EZN, with overall survival (OS). Automated BSI generated by EXINI bone^BSI platform is quantitative representation of tumor burden as a percent of total skeletal mass. Bivariate cox regression analysis was used to evaluate the association of BSI and PSA with OS. Results: Thirty-five pts, who initiated ENZ treatment at (SUH), were eligible for the BSI analysis. Follow-up scans for the BSI analysis were available from 24 pts. Median baseline BSI value was 2.92 (range: 0.0 - 11.72) and at follow-up the median BSI value was 2.83 (range: 0.0 - 12.65). OS was associated with BSI at both baseline and at follow-up as opposed to that of PSA (table 1). The change in BSI between baseline and follow-up was also significantly associated with OS, whereas the change in PSA was not. Conclusions: Automated BSI and its relative change were observed to be associated with OS in mCRPC pts receiving ENZ as standard of care treatment. The result deserves further validation, in controlled investigational studies, of BSI as a quantitative imaging biomarker indicative of efficacy response to second-line treatment in mCRPC pts.