In a retrospective study of 30 cases with fatal basilar artery thrombosis we compared clinical and autopsy findings for clinicopathological correlation. There are only few clinical symptoms which point to a definite localisation and extent of thrombosis or cerebral infarction. In most cases loss of consciousness and hemi- or tetraparesis are due to pontine or mesencephalic infarction. Palsy of the facial nerve suggests infarction of the pons. The incidence of cranial nerve involvement is too low to allow localisation of infarction. There was no case showing infarction of the diencephalon or the medulla oblongata when pontine or mesencephalic structures were intact. Moreover, these infarctions remained clinically silent. No clinical predictors of outcome could be defined. Even in patients without detectable impairment of consciousness or stenosis of the vertebral arteries, the disease may prove fatal.