Low molecular weight heparins (LMWHs) are as efficient as unfractionated heparin (UFH) for prevention and treatment of thromboembolism. There is no evidence that monitoring the dose improves the clinical efficacy. In contrast, any overdosage increases the risk of hemorrhage. Because renal function plays a significant role in the elimination of LMWH, curative treatment should be monitored with an anti-factor Xa assay in patients presenting renal insufficiency, in the elderly, and in patients presenting an increased hemorrhagic risk. It is advisable to sample the patient at peak activity (3 to 5 hours after the subcutaneous [sc] administration) and to target the mean anti-factor Xa activity that was found efficient and safe in the clinical trial. This target is different for each LMWH and each dose regimen.