The aim of the study is to better define transcranial Doppler (TCD) criteria for posterior circulation vasospasm. Basilar artery (BA) diameters were measured and compared with diameters obtained from baseline arteriograms in 144 artheriographies done in patients with aneurysmal subarachnoid haemorrhage (SAH). Both BA and extracranial vertebral artery (ECVA) flow velocities (FV) were measured by TCD in order to obtain an intracranial/extracranial flow velocities ratio. The velocity ratio between the basilar artery and the extracranial vertebral arteries (BA/VA) strongly correlated with the degree of BA narrowing (p<0.0001). A ratio higher than 2.0 was associated with 73% sensitivity and 80% specificity for BA vasospasm. A ratio higher than 2.5 with BA velocity greater than 85 cm/sec was associated with 86% sensitivity and 97% specificity for BA narrowing of more than 25%. A BA/VA ratio higher than 3.0 with BA velocities higher than 85 cm/sec was associated with 92% sensitivity and 97% specificity for BA narrowing of more than 50%. Our data show that the BA/VA ratio improves the sensitivity and specificity of TCD detection of BA vasospasm.