Background: Venous thromboembolism (VTE) is the most common non-surgical complication after major pelvic surgery. Little is known about the risk factors or the time of development of postoperative venous thrombosis. Methods: A cohort of 523 consecutive patients undergoing radical prostatectomy with lymphadenectomy was prospectively assessed by complete compression ultrasound at days -1, +8 and +21. Results: Complete data were available in 415 patients, while four patients had VTE before surgery and were excluded from the analysis. In the remaining 411 patients, 71 VTE events were found in 69 patients (16.8%). Most were limited to calf muscle veins (56.5%), followed by deep calf vein thrombosis (23.2%), proximal deep vein thrombosis (DVT, 14.5%) and pulmonary embolism (PE, 5.8%). Of the 14 patients with proximal DVT/PE, 11 patients (78.6%) developed VTE between days 8 and 21. Risk factors for VTE were a personal history of VTE (OR 3.0), pelvic lymphoceles (LCs) impairing venous flow (OR 2.8) and necessity of more than two units of red blood cells (OR 2.6). Conclusion: Venous thromboembolism is common after radical prostatectomy. A significant proportion develops after day 8, suggesting that prolonged heparin prophylaxis should be considered. Since LCs with venous flow reduction result in higher rates of VTE, hemodynamically relevant lymphoceles should be surgically treated.