We reviewed open lung biopsies from 35 patients with Wegener's (pathergic) granulomatosis in order to study the histogenesis of the pulmonary lesions and to identify the early lesions. The process of pathergic necrosis is fundamental in the production of extravascular and vascular lesions and was divided into micronecrotic and macronecrotic types. Micronecrosis, usually with neutrophils (microabscesses), constitutes the early phase in the development of the pathognomonic organized palisading granuloma. The palisading granuloma differs from the compact granuloma of tuberculoid type, which occurs in infections and sarcoidosis but not in Wegener's (pathergic) granulomatosis. There is a progression of disease from micronecrosis to macronecrosis (widespread necrosis) and then to fibrosis. Macronecrosis surrounded by palisading histiocytes or diffuse granulomatous tissue indicates active disease, whereas necrosis surrounded by fibrous tissue indicates previously active disease. Most cases have a combination of micronecrosis, and fibrosis. We established the relative diagnostic value of various histologic features. Arteritis and phlebitis as classically described in Wegener's granulomatosis were present in most but not all cases. We believe that Wegener's granulomatosis primarily affects both vascular and extravascular collagen and reticulum and that vasculitis represents a primary necrosis of walls of blood vessels. We believe that the concept of Wegener's granulomatosis as a vasculitis is too restrictive and does not include many cases with only extravascular histologic changes.