Noninfectious aortitis typically involves the ascending aorta and causes aneurysms that result in aortic root repair. Aortitis is clinically categorized into groups that include Takayasu disease, giant cell aortitis, and isolated aortitis. We present a histopathologic classification of 52 patients with aortitis, without reference to clinical findings, which are often unknown to the diagnostic pathologist. The largest group (43 patients) was designated necrotizing aortitis (NA), characterized by zonal medial laminar necrosis, rimmed by giant cells. Healed areas were common and were characterized by extracellular accumulation of proteoglycans imparting the appearance of medial degeneration. NA had a bimodal age distribution with a separation at age 65 (adult NA versus elderly NA). Adult NA (24 patients; 50% female; age range, 24-60) was generally isolated, but 2 patients had associated autoimmune disease (Crohn disease and lupus erythematosus, respectively). Elderly NA (19 patients; 94% female; age range, 68-80) was likewise usually isolated, but 1 patient had temporal giant cell arteritis and 1 seronegative arthritis. Subsequent complete rheumatologic workup on 17 patients with NA was negative. Adult NA differed significantly from elderly NA (fewer women, P = .002; greater adventitial scarring, P = .007). The second group of aortitis was designated non-NA (NNA), characterized by the absence of necrosis, with diffuse medial inflammation. The NNA group was composed of 3 men and 6 women, all older than 65 years (mean, 72 +/- 6 years). Four had a history of temporal arteritis. NNA patients differed from elderly NA (more frequent temporal arteritis, P = .03; less medial destruction and proteoglycan deposits, P < .01; increased medial T-lymphocytes, P = .05; and more frequent dissection, P = .002). We conclude that NA is usually isolated, has distinct histologic features based on age less than or more than 65 years, and is clinicopathologically distinct from NNA. NNA is less often isolated and best classified as giant cell aortitis. Adult NA has histologic features classically associated with Takayasu disease but is limited primarily to the ascending aorta and has no sex predominance.