Headache can be an invaluable premonitory signal of imminent subarachnoid hemorrhage and cerebral infarction and can herald the onset of ominous and sometimes elusive disorders (arterial dissection, encephalitis, systemic and central vasculitides, and cerebral venous thrombosis) which have the potential for neurologic catastrophe and are often not obvious on routine CT brain imaging. Only rarely does serious underlying disease give rise to a headache that exactly mimics a migraine or tension headache. Inevitably, there are atypical features or warning signals. A limited number of serious causes for headache which may be "CT-negative" should be considered in patients with "red flag" manifestations, such as seizures and cognitive changes. These should prompt further investigation with MRI and/or lumbar puncture.