Although it is well known that Wallenberg's syndrome is caused by occlusion of the vertebral artery (VA) or the posterior inferior cerebellar artery (PICA), the etiology of the occlusion is rarely documented. During the course of Wallenberg's syndrome, patients often complain of headache. We thought that these headaches might be caused by dissecting aneurysm (DA) of the vertebral artery, and so we studied the incidence of DA in our cases with Wallenberg's syndrome. Although many variants exist, Wallenberg's syndrome encompasses several neurological symptoms due to a disorder of the nucleus and nerve tracts located in the lateral part of the medulla. We diagnosed our patients as having Wallenberg's syndrome on the basis of symptoms such as loss of pain and temperature sensation in the unilateral face and contralateral body, cerebellar ataxia, and dysphasia. We investigated 22 cases of Wallenberg's syndrome over a five-year period, and excluded patients who developed subarachnoid hemorrhage upon onset of the syndrome. Our cases can be divided into two groups; one with severe stenosis or occlusion of VA (n = 15) and the other with occlusion of PICA (n = 5). The angiograms of the two remaining patients showed no abnormal findings. The mean age of the VA group (42.5 yrs.) was younger than that of the PICA group (64.2 yrs.). The age distribution of the PICA group is similar to that of other occlusive cerebrovascular diseases. Seven cases of the VA group demonstrated aneurysmal dilatation and luminal stenosis, and so they were diagnosed as having dissecting aneurysm of VA.(ABSTRACT TRUNCATED AT 250 WORDS)