The diagnosis of vasculitis is first and foremost a clinical one. Correct diagnosis requires a high index of suspicion coupled with knowledge of the manifestations of other disorders that may masquerade as vasculitis. Treatment of vasculitis requires prolonged use of drugs with the potential for serious side effects. Whereas the prompt initiation of definitive treatment is a very high priority, there is also substantial risk of inappropriately treating self-limited and more benign disorders mimicking vasculitis. This has been a particular problem with primary angiitis of the central nervous system. Laboratory studies, particularly tissue biopsy, provide a crucial adjunct to clinical diagnosis.