The most common complication of heparin therapy is bleeding. Patient actors that increase the risk of bleeding include recent surgery or trauma, dvanced age (ie, age older than 65 years), alcoholism, aspirin use, bed est, immobility, obesity, congestive heart failure, cancer, hypercoagulale states (thrombophilia), childbirth, and pharmacologic doses of estroens. There is yet a clear association between subtherapeutic partial hromboplastin time (PTT) and bleeding complications. The hemorrhagic omplications have been mostly linked to the underlying clinical risk actors rather than PTT levels being above the therapeutic range. The rate of major bleeding associated with anticoagulant use varies rom 1% to 33%. The reason for such a wide range was mainly the ariation of criteria for severe and minor bleeding. According to ecently published studies, the rate of major hemorrhage with unfractionted heparin ranges from 0% to 7%. Fatal bleeding with a 5to 14-day ourse of heparin ranges from 0% to 2%. A meta-analysis of published tudies shows that the average daily frequencies of fatal and major leeding during heparin therapy were 0.05% and 0.8%, respectively. The management of bleeding in a patient receiving heparin depends on he location and severity of bleeding, the risk of recurrent venous hromboembolism, the level of the PTT, and the anti-Xa level. Importantly, one should monitor the effect of low molecular weight eparin (LMW heparin) via anti-Xa levels in patients with renal dysfuncion owing to renal clearance.