Five patients, three women aged 87, 50, and 31 years, and two men aged 31 and 32 years, presented with severe headache of sudden onset. A sudden onset of unusually severe headache is suggestive of an intracranial haemorrhage or other serious disease, even in the absence of focal neurologic deficits. The diagnoses were subdural haematoma, cerebral venous sinus thrombosis, idiopathic thunderclap headache, subarachnoid haemorrhage, and viral meningitis, respectively. There are no characteristics from history or examination that accurately discriminate among all these causes; idiopathic thunderclap headache and subarachnoid haemorrhage are commonest. Consultation of a neurologist and further ancillary investigations are necessary for proper diagnosis and treatment.