Low molecular weight heparin has greater advantages over unfractionated heparin. It is more bioavailable, and laboratory monitoring is not necessary. Compared with unfractionated heparin, low molecular weight heparin does not result in increased risk of major bleeding. However, the bleeding tendency is not predictable in patients with renal failure, because elimination of low molecular weight heparin is delayed and it does not alter prothrombin times or partial thromboplastin times. Recently, we experienced two cases of enoxaparin-associated retroperitoneal hematoma in chronic dialysis patients. A 57- year-old woman developed retroperitoneal bleeding, during treatment with enoxaparin(1 mg/kg q 12 hours) and oral aspirin. The other patient, a 49- year-old man developed retroperitoneal hematoma after discontinuation of enoxaparin and aspirin. Both patients had inguinal pain, femoral neuropathy, anemia and hypotension. They recovered gradually and their hematoma size were decreased by conservative treatment. These results suggest that anti-Xa acivity monitoring may be warranted in renal insufficiency patients who are receiving low molecular weight heparin If anti-Xa activity test is not available, unfractionated heparin could be used with monitoring of activated partial thromboplastin time. And the possibility of retroperitoneal hematoma should be considered, whenever the acute symptoms including inguinal pain, leg pain, anemia, or hypotension occured during the anticoagulation therapy.