A retrospective case note survey of chronic subdural haematomata was carried out in an attempt to throw some light on the difficulties encountered in clinical diagnosis. The combination of raised intracranial pressure headache, fluctuating drowsiness and mild hemiparesis, although highly suggestive of subdural haematoma, is not always encountered, and epilepsy, aphasia, hemianopia and dense hemiplegia can all occur contrary to 'text book' descriptions. Head injury or other aetiological factors are commonly absent. The presentation may mimic tumour, dementia, cerebrovascular accident or subarachnoid haemorrhage. Non-invasive investigations may yield false negative results, although in the case of radionucleide scanning and computerized axial tomography the reliability is approaching 90 per cent. The diagnosis will, however, remain an unexpected finding at angiography in a percentage of cases.