Forty-nine patients with clinical features of vascular disease in the vertebrobasilar territory were examined to evaluate the diagnostic performance of computed tomography. It provided useful information in the diagnosis of acute vascular lesions in patients in whom a definite episode had occurred not longer than 2 weeks before computed tomography was undertaken. It was of value in both haemorrhagic and thromboembolic groups but in many patients angiography was required to make a substantive diagnosis. It was of less value in patients with vague and ill defined symptoms and of no value in transient ischaemia.