Immediate anticoagulant treatment is essential to reduce morbidity and mortality in patients with acute venous thromboembolism (VTE). Currently, rapid anticoagulation can only be achieved with parenteral anticoagulants, such as heparin or low-molecular-weight heparin (LMWH). Weight-adjusted LMWH is the treatment of choice, because it produces predictable anticoagulation and does not require coagulation monitoring. If heparin is used, the activated partial thromboplastin time must be monitored and the heparin dose adjusted to ensure a therapeutic level of anticoagulation. Heparin is recommended for patients with renal impairment and for those at high risk of bleeding. The selective factor Xa inhibitor fondaparinux is a recently introduced alternative to heparin or LMWH for initial VTE treatment. Heparin, LMWH, or fondaparinux should be given for at least five to seven days. Vitamin K antagonists should be initiated on the first day, or as soon as possible, in patients who are candidates for an oral anticoagulant. An oral anticoagulant agent to be used without laboratory monitoring for both acute and long-term treatment of VTE remains an unsolved clinical need in the treatment of VTE.