Background: Most pts with bladder cancer will be treated with immunotherapy. There is concern for increased TE risk with CPIs in this already high risk population. We present the first analysis of the incidence and outcomes of venous (VTE) and arterial (ATE) thromboembolism in pts with bladder cancer treated with CPIs. Methods: Consecutive pts with bladder cancer treated with CPIs at the Cleveland Clinic from 1/2015 to 12/2019 were identified and TE events noted. Overall survival (OS) was estimated using Kaplan-Meier method and the impact of VTE on OS was evaluated using Cox proportional hazards regression. Results: Of 274 pts, 72% were men (median age 73.3 years, 89% white), 82% had pure UC, 92% had lower tract disease, and 67% had a Bajorin score ≥1 (median KPS 90, 61% visceral metastases), 59% had prior systemic therapy (median 1, range 0-4) and 36% had prior TE (14% ATE, 19% VTE, 0.4% both). At CPI initiation, 24% were on antiplatelet therapy, and 15% on therapeutic anticoagulation. CPI (median doses 5, range 8.5-59) included: 40% atezolizumab, 3% nivolumab, 57% pembrolizumab. VTE occurred in 14% (n = 37), including 8% DVT, 4% PE, 2% both. DVT locations were 56% lower limb, 26% upper limb, 15% visceral vein, 4% visceral+upper limb. 2% (n = 5) had ATE (1% CVA, 0.4% visceral, 0.4% left subclavian). 92% of VTE and all ATE occurred within 6 months of CPI initiation. The incidence of TE was 10.9% (95%CI 6.6%—15.1%) at 6 months and 19.8% (95%CI 13.3%-26.4%) at 12 months. 82% of VTE (mean 6 days) and all ATE (mean 5 days) resulted in hospitalization. Multivariate analysis showed TE (HR 2.296, 95%CI 1.451-3.632, p = 0.0004), Bajorin score 1 (HR 1.490, 95%CI 1.036-2.142, p = 0.0315), and Bajorin score 2 (HR 3.50, 95%CI 2.14-5.74, p < 0.0001) were independently associated with worse OS. Conclusions: CPIs in bladder cancer pts are associated with a high TE risk, especially within six months of initiation. TE is associated with worsened survival. Further investigation into the risk factors for CPI-associated TE is needed to identify if benefits exist from thromboprophylaxis.