The association between cancer and activation of blood coagulation has been described since Trousseau's time. The hypercoagulable state often encountered in cancer patients not only acts as an important risk factor for thrombosis, but also may play a role in tumor progression and metastasization. An antineoplastic effect of anticoagulants in this setting has often been hypothesized. The results of recently conducted clinical trials suggest that cancer patients could have a benefit from administration of low molecular weight heparin, particularly in those with nonadvanced disease. Additional clinical trials are needed to provide an important step forward in this challenging setting.