Twelve cases of pathergic (Wegener's) granulomatosis are described, with special attention focused on the long duration of mucosal and skin lesions in untreated cases, designated as the protracted superficial phenomenon, and on the histologic features that may be helpful in making the diagnosis. The long duration, often the result of a lack of proper interpretation of histologic details, was associated in some of the cases studied with the development of intractable renal failure or mutilation of the face. Since cytotoxic therapy offers the opportunity to prevent these complications, the desirability of an early diagnosis is obvious. Biopsy is the principal means of diagnosis, and therefore interpretation of histologic details is of paramount importance. Helpful histologic features found in the extravascular and vascular tissues of the specimens studed were focal necrosis, fibrinoid degeneration, palisading granulomas, giant cells, and vasculitis. Nonpalisading foci of granular necrosis or fibrinoid degeneration appeared to precede the development of the typical palisading granuloma. Both focal necrosis and focal fibrinoid degeneration occurred independently of intrinsic vascular involvement and in themselves are distinctive features of pathergic (Wegener's) granulomatosis. There was predominance of the extravascular components in the cases studied with occasional absence of vasculitis. Both the extravascular and vascular components are important in making a definitive diagnosis, but the extravascular component is characteristic, even in the absence of vasculitis. The extravascular tissues and the vessels are parallel contemporaneous target tissues.