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e17001 Background: Prostate cancer is thought of as an immunologically cold tumor, however a small proportion of patients treated with immunotherapy have impressive responses. Studies into the effect of autoimmune disease on risk of prostate cancer have yielded contradictory results and little is known about the survival of patients with concurrent autoimmune disease and prostate cancer. This study compared outcomes in patients with prostate cancer with and without autoimmune disease. Methods: This study was a retrospective analysis of patients from the SEER-Medicare databases from 2007-2014 with prostate cancer. Patients with a history of autoimmune disease were identified using ICD-9 codes. The effects of autoimmune disease on overall survival (OS) and cancer-specific survival (CSS) were estimated using multivariable Cox regression and Gray’s method respectively controlling the effects of age, race and chronic kidney disease (CKD). The cumulative CSS was estimated taking death as a competing risk. Results: The overall prevalence of investigated autoimmune diseases among the 172,061 patients with prostate cancer was 23.74%. The most common autoimmune diseases identified were rheumatoid arthritis (RA) (20.93%), psoriasis (2.43%) and ulcerative colitis (UC) (0.91%). In stage IV prostate cancer, OS (p = 0.018) and CSS (p < 0.001) were significantly higher in patients with autoimmune disease, with a median OS of 55 months compared to 48 months in patients without autoimmune disease. After adjusting for the effects of age, race, and CKD, autoimmune disease was still predictive of higher OS (HR: 1.41, 95% CI: 1.33 – 1.5, p < 0.0001) and CSS (HR: 1.30, 95% CI: 1.21 – 1.39, p < 0.0001). Patients with autoimmune disease and stage II and III prostate cancer had lower OS (p values < 0.0001) compared to patients without autoimmune disease. Conclusions: The study showed higher prevalence of RA, Crohn disease, UC, and systemic lupus erythematosus in patients with prostate cancer compared to cohorts of similar age ranges in the general population. History of autoimmune disease predicted significantly higher OS and CSS in patients with stage IV prostate cancer even when age, race and CKD were controlled for. It is possible that an improvement in OS with a history of autoimmune disease was evident in stage IV but not in stages II and III because anti-tumor autoimmune activity plays a larger role when prostate cancer effects regional lymph nodes or has distant metastases.