Clinical and histopathologic features of 101 cases of necrotizing vasculitis were selected on the basis of the following histopathologic criteria: fibrinoid necrosis of blood vessel walls, endothelial cell hyperplasia, and an infiltrate within and around the blood vessel walls predominantly of polymorphonuclear leukocytes. There were three clinical patterns of vasculitis: (1) associated with other coexistent disease, (2) associated with known precipitating events, and (3) idiopathic. Two histologic features were particularly notable in view of the clinical findings. First, vasculitis extending deep into the reticular dermis or subcutaneous tissue seemed to be associated more often with systemic disease such as malignancy or connective tissue disease. Second, in biopsy specimens from patients with hypocomplementemia, the inflammatory infiltrate was composed almost exclusively of neutrophils, as compared with the mixed infiltrate seen in normocomplementemic vasculitis.