The rationale for thrombophrophylaxis is well known. Without prophylaxis the incidence of objectivlely confirmed, hospital-acquired deep venous thrombosis is approximately 10% to 40% among medical or general surgical patients. In urology the prevalence is 15-40%. The Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy1 recommend against specific prophylaxis in patients undergoing transurethral procedures. Our study is the first double-blind, placebo-controlled study with Dalteparine as thrombophrophylaxis in transurethral surgery. The aim of the study is to proof that thrombophrophylaxis causes not more complications in transurethral surgery than without.