Seven cases of aneurysm of the vertebral and basilar arteries, in which the clinical features suggested a diagnosis of posterior fossa tumour, are presented. Their clinical manifestations and the difficulties of diagnosis are discussed. These lesions may mimic cerebellopontine angle tumours or brain-stem gliomas, or, less commonly, the picture is one of a compressive lesion at the foramen magnum. The need for vertebral angiography is emphasized. Exploratory surgery carries a high risk, and is to be avoided.