Basilar artery thrombosis is a potentially treatable neurologic disease whose initial symptoms are sometimes difficult to recognize. The basilar artery is hyperdense on computed tomography in cases with basilar artery thrombosis. We have studied computed tomography attenuation values before and after contrast administration in a total population of 117 patients, 12 of whom were diagnosed angiographically as having basilar artery thrombosis. While it is not always possible to detect basilar artery thrombosis simply by evaluation of the tomodensitometric values before or after contrast, the differential values obtained by subtracting them (after contrast/before), or by an attenuation ratio corresponding to the differential value divided by the value before contrast, are more helpful. Indeed, the differential values and attenuation ratios in the normal population have a mean value of 51.1 and 1.37 compared with 14.47 and 0.33 in patients with basilar artery thrombosis.