Several case-control studies have reported enhanced platelet activity, hypercoagulation and/or reduced fibrinolytic activity in patients with acute ischemic stroke. However, results of these studies are conflicting and do not allow to make recommendations regarding heparin treatment. The aim of heparin treatment in stroke patients is to prevent venous thromboembolic complications, to improve patient neurologic outcome, to reduce mortality and to prevent early recurrence. Unfortunately, only the first objective has been confirmed. As far as neurologic outcome and mortality are concerned, only trials performed since the CT era should be taken into account because the former ones did not rule out cerebral haemorrhages. The most recent clinical trials using LMWHs gave better results than the previous trials with UFH, but data are still conflicting and firm recommendations cannot be made until the results of ongoing megatriasl (such as IST) become available. As regards prevention of early recurrence, most authors agree that heparin is indicated in cardioembolic stroke.