Hemorrhagic infarction and cerebral hematoma are feared events that may follow cerebral ischemia. Newly developed thrombolytic agents may be effective stroke therapy, but may also promote hemorrhagic complications after ischemic stroke. It is therefore critically important to understand the true incidence of hemorrhagic transformation after ischemic stroke, and to identify if possible the mechanisms underlying the phenomenon. In recent years, studies using serial computed tomography to identify hemorrhage have shown that transformation occurs in 15 to 43% of patients presenting with ischemia. Experimental and clinical evidence support the notion that hemorrhage results from augmented collateral circulation into the ischemic zone, perhaps in concert with hypertension. Recanalization and distal migration of the thrombus are not factors that are associated with transformation. Pharmacologic recanalization using thrombolytic drugs are not likely to be associated with hemorrhage if reperfusion is accomplished very soon after the onset of neurologic symptoms.