Intensivists are sometimes faced with unexplained neurologic defects in ICU patients. A subacute presentation over weeks or months characterized by headache and mental status change with focal deficits in the absence of evidence of secondary vasculitis or other diseases mentioned in the differential diagnosis should arouse suspicion of PACNS. Delay in diagnosis of this rare condition may lead to additional morbidity and prolong ICU stay. There is also a risk of permanent cognitive dysfunction with untreated PACNS. A reactive CSF picture, ischemic changes on MR imaging, and alterations in vessel caliber on cerebral angiography are not diagnostic but strengthen the evidence for PACNS. A brain biopsy may be required to confirm the diagnosis. High-dose steroid therapy with a prolonged course and gradual taper controls the disease in most cases. Additional immunosuppressive therapy is needed in some patients.