This study considers diagnostic problems in the case of a 52 year old patient presenting with left sided headache and Horner's syndrome. Color Doppler sonography of the internal carotid artery with transcranial duplex sonography showed decreased blood flow velocity and increased impedance of flow in the left internal carotid artery. Flow velocity in the left middle cerebral artery was moderately decreased whereas flow in the proximal segment of the anterior cerebral artery and in the posterior communicans artery was re-directed towards the left internal carotid artery. These findings indicated the need for cerebral angiography which revealed that a vast section of the distal segment of the left internal carotid artery was dissected. Pharmacological treatment with low molecule heparine brought prompt resolution of the signs and symptoms as well as normalization of the blood flow parameters, as examined with Doppler ultrasonography Color Doppler ultrasonography would appear to have high utility in the initial diagnosis of internal carotid artery dissection and the usefulness of this method in the monitoring of treatment.