Objective. To describe the clinical, biologic, and histologic features of temporal artery biopsy (TAB)-localized systemic necrotizing vasculitides (SNV), and to assess their frequency among elderly patients undergoing TAB for suspected giant cell (temporal) arteritis (GCA).Methods. The frequency of a TAB localization of SNV was prospectively assessed in a multicenter study of elderly patients undergoing TAB for suspected GCA. All patients with SNV fulfilling the American College of Rheumatology criteria for a specific vasculitic syndrome and with evidence of vasculitis on TAB were included in a retrospective, descriptive study.Results. SNV was diagnosed based on the TAB in 1.4% of the patients with suspected GCA and in 4.5% of the positive (inflamed) TAB specimens. We retrospectively selected 27 patients (18 female, 9 male; mean ؎ SD age 62 ؎ 15 years, range 22-79 years) with SNV and TAB-localized vasculitis. Only 2 of these patients were known to have SNV before TAB localization. Twenty-two patients (81%) had cephalic symptoms, including jaw claudication in 33%, clinically abnormal temporal arteries in 33%, and neuro-ophthalmologic symptoms in 11%. All patients had systemic symptoms suggestive of SNV and histologically proven NV in the TAB specimens (70%) or elsewhere in other biopsy sites (74%). Abnormal biologic results suggestive of SNV were present in 17 patients (63%). For 4 patients, the TAB-documented involvement led to initial misdiagnoses of GCA, and systemic manifestations that developed under steroid therapy revealed the correct diagnosis. The final diagnoses of the patients were polyarteritis nodosa (PAN) (n ‫؍‬ 11), Churg-Strauss syndrome (n ‫؍‬ 6), micropolyangiitis (n ‫؍‬ 3), Wegener's granulomatosis (n ‫؍‬ 3), hepatitis B virus-related PAN (n ‫؍‬ 2), hepatitis C virus-related cryoglobulinemic vasculitis (n ‫؍‬ 1), and rheumatoid vasculitis (n ‫؍‬ 1).Conclusion. TAB-localized SNV presents a major diagnostic dilemma because it can mimic GCA. Careful analysis of clinical, biologic, and histologic data should lead to the correct diagnosis and help guide the clinician's choice of appropriate therapy.