Anticoagulation with heparin has a valuable place in prevention and management of deep venous thrombosis. However, the benefit of heparin in acute ischemic stroke and transient ischemic attack remains unclear despite its widespread use for these indications. Heparin also carries several risks, including unpredictable anticoagulation effects, bleeding, and thrombocytopenia. Low-molecular-weight heparins (LMWHs) and heparinoids have several advantages over heparin, such as higher bioavailability, more predictable anticoagulant effects, and less interaction with platelets. Heparin, LMWHs, and heparinoids have been studied in acute ischemic stroke with variable results. Of three recent, large, controlled clinical trials, only one documented a net benefit of treatment. Fewer patients treated with an LMWH within 48 hours of stroke were dead or disabled at 6 months compared with placebo-treated patients. The largest randomized clinical trial of heparin in acute stroke (the International Stroke Trial) showed that heparin was associated with a significant excess in bleeding complications but no clinical benefit at 6 months. Interim analysis of the TOAST (Trial of ORG 10172 in Acute Stroke Treatment) study also showed an excess number of bleeding complications in the treated group without a corresponding benefit on stroke outcome at 3 months. Therefore, although heparin, LMWHs, and heparinoids continue to be used in the management of patients with acute ischemic stroke, their value in recurrent stroke prevention and in the treatment of stroke-in-progress remains unsettled. Ongoing studies may help to clarify the use of LMWHs and heparinoids in these patients.