Birnbaum and Hellmann Article review their experience with primary angiitis of the central nervous system (PACNS). The clinical features, complex and difficult differential diagnosis, and current therapies are reviewed. However, most patients thought to have PACNS have a different disorder. Most interpretations of angiograms as being highly suggestive of PACNS will be owing to vasospasm. This and other compelling issues of diagnosis are amplified and defined.