CONTEXT:Pharmacological thromboprophylaxis involves balancing a lower risk of venous thromboembolism (VTE) against a higher risk of bleeding, a trade-off that critically depends on the risks of VTE and bleeding in the absence of prophylaxis (baseline risk). OBJECTIVE:To provide estimates of the baseline risk of symptomatic VTE and bleeding requiring reoperation in urological cancer surgery. EVIDENCE ACQUISITION:We identified contemporary observational studies reporting symptomatic VTE or bleeding after urological procedures. We used studies with the lowest risk of bias and accounted for use of thromboprophylaxis and length of follow-up to derive best estimates of the baseline risks within 4 wk of surgery. We used the GRADE approach to assess the quality of the evidence. EVIDENCE SYNTHESIS:We included 71 studies reporting on 14 urological cancer procedures. The quality of the evidence was generally moderate for prostatectomy and cystectomy, and low or very low for other procedures. The duration of thromboprophylaxis was highly variable. The risk of VTE in cystectomies was high (2.6-11.6% across risk groups) whereas the risk of bleeding was low (0.3%). The risk of VTE in prostatectomies varied by procedure, from 0.2-0.9% in robotic prostatectomy without pelvic lymph node dissection (PLND) to 3.9-15.7% in open prostatectomy with extended PLND. The risk of bleeding was 0.1-1.0%. The risk of VTE following renal procedures was 0.7-2.9% for low-risk patients and 2.6-11.6% for high-risk patients; the risk of bleeding was 0.1-2.0%. CONCLUSIONS:Extended thromboprophylaxis is warranted in some procedures (eg, open and robotic cystectomy) but not others (eg, robotic prostatectomy without PLND in low-risk patients). For "close call" procedures, decisions will depend on values and preferences with regard to VTE and bleeding. PATIENT SUMMARY:Clinicians often give blood thinners to patients to prevent blood clots after surgery for urological cancer. Unfortunately, blood thinners also increase bleeding. This study provides information on the risk of clots and bleeding that is crucial in deciding for or against giving blood thinners.