Immune-mediated heparin-induced thrombocytopenia (HIT) is a rare but serious side effect of heparin therapy which presents various thromboembolic events associated with high mortality and morbidity. There have been few reports about the prevalence of HIT in acute ischemic stroke, which our retrospective study and a multi-center prospective cohort study respectively estimated as 0.5% and 1.7% of unfractionated heparin-treated acute ischemic stroke patients. Once the onset of HIT is suspected, its therapy should be started immediately because treatment delay of HIT will bring a poor outcome. Stroke physicians should be aware of HIT, as heparin use will become increased more than ever in clinical practice with the development of new intravascular treatment techniques.