e18786 Background: The share of oncology practices that were vertically integrated with hospitals more than doubled in the past decade. Vertical integration leads to higher spending, but evidence on quality and outcomes remains inconclusive. This study examines how integration between hospitals and oncologists affects patient outcomes and spending among patients with metastatic prostate cancer. Methods: Using the SEER-Medicare linkage with the Medicare Data on Provider Practice and Specialty file, we identified Medicare beneficiaries diagnosed with metastatic, castration-resistant prostate cancer who initiated chemotherapy between 2008-2017 (n = 9,167). We used a claims-based approach to identify treating oncologists and their integration status (as indexed by percent of total services billed to hospital outpatient departments). Outcomes included: 1) time on chemotherapy, 2) survival from chemotherapy initiation, 3) supportive care use (i.e. bone-modifying agents [BMA]), 4) Medicare payment for the first 3 months of chemotherapy initiation and last 3 months before end of life (EOL). The differences-in-differences approach was used to compare patient outcomes before and after oncologists became integrated versus those whose oncologists remained non-integrated or always integrated. Weibull survival and linear models were used for regression analysis, adjusting for sociodemographic, clinical, and market characteristics. Results: The proportion of patients treated by integrated oncologists increased from 28% in 2008 to 55% in 2017. Vertical integration was associated with 13.6 percentage points increase (95% CI = 6.1 to 21.1) in the likelihood of BMA use, but no significant changes for time on chemotherapy and survival. Average Medicare spending in the first 3 months of chemotherapy initiation and last 3 months of EOL was $34,324 and $42,624, respectively. Vertical integration did not have a significant effect on overall spending. Further decomposition showed decreases in professional service costs (-$4,264, 95% CI = -$6,642 to -$1,887) along with an increase in outpatient care costs ($3,941, 95% CI = $159 to $7,723) for the first 3 months of chemotherapy. A similar pattern was found for spending in the last 3 months of EOL. Conclusions: Vertical integration of oncologists was associated with increased use of supportive care, but not time on chemotherapy or survival among metastatic prostate cancer patients. Although oncologists switched service billing from physician offices to hospital outpatient departments, we did not detect significant increases in total Medicare spending after vertical integration. This suggests the potential for quality improvement following vertical integration without substantial increases in healthcare spending. Future studies should extend the investigation to other cancer types and patient outcomes.