253 outpatients were included in a prospective randomized study. All patients had incidences of injuries of the lower limb immobilized by a plaster cast. 126 patients (group I) received a subcutaneous injection of Fraxiparin daily, and 127 patients (group II) received no thromboprophylaxis. Without prophylaxis 21 (16.5%) cases developed a thrombosis, and with prophylaxis a thrombosis could be proven in 6 (4.8%) patients only (p < 0.01). Thus, for all patients with an injury of the lower limb being immobilized by a plaster cast, a thromboprophylaxis with a low-molecular-weight heparin is recommended.