This is a review of a double-blind, prospective study comparing the thromboprophylactic efficacy and safety of two different prophylactic regimens of a low molecular weight heparin (tinzaparin) in 250 consecutive patients (aged < or 18) undergoing primary elective hip arthroplasty. Regimen 1: 75 U anti-Xa/kg BW (actual range 63 to 91) once daily started 12 hours before operation; and regimen 2: 50 U anti-Xa/kg BW (actual range 41 to 71) once daily started 2 hours before operation. Both regimens were administered in a weight-adjusted fashion and were continued for 7 days after operation or until full mobilization. Efficacy was evaluated by occurrence of postoperative deep vein thrombosis (DVT) diagnosed by bilateral ascending phlebography on day 7 +/- 2 after operation, and the venograms were evaluated in an assessor blind fashion by a panel of three expert radiologists. Safety was evaluated by the amount of blood lost and transfusion requirements during and after the operation; all bleeding complications, reoperations, adverse events and deaths were observed during the study. A 3-month follow-up on survival and occurrence of thromboembolism was performed on all randomized patients. The result was a significantly better protective effect against proximal DVT by regimen 1 compared with regimen 2. This was achieved with improved safety in terms of a significantly decreased need for blood transfusions during operation and fewer wound complications in the postoperative period in favor of regimen 1. Therefore, tinzaparin administered in a dosage of 75 U anti-Xa/kg BW 12 hours before surgery is significantly more protective against proximal DVT and safer than the standard regimen of 50 U anti-Xa/kg BW started 2 hours before surgery in patients undergoing primary elective hip arthroplasty.