The easily accessible and available PCT reliably identifies reversible and irreversible ischaemia in acute stroke patients. These knowledge will allow treatment strategies to become more appropriate and individualized. Patients with significant penumbra may be candidates for treatment with dangerous or costly medication, and patients without may not, independently of duration of stroke symptoms. Furthermore, PCT also has the scientific potential to identify appropriate patients for therapeutic trials. Finally, salvage of PCT-defined penumbra could be used as a surrogate marker for effectiveness of interventions.