Table 1 summarizes some of the differentiating characteristics among these diseases. Clinically, the differentiating features are that classic Wegener's granulomatosis involves both the upper and lower respiratory systems and kidneys. Renal involvement is the major cause of morbidity and mortality. Churg-Strauss syndrome has the distinguishing characteristics of an allergic asthmatic prodrome and a profound eosinophilia. It encompasses and destroy's tissue not only of the lungs but also most of the other organ systems in the body. A major cause of mortality is cardiac disease, which is not a feature of either of the other two diseases. Finally, lymphomatoid granulomatosis is seen predominantly in the lungs, skin, and central nervous system and is the only one of the three that is associated with the development of a lymphoma. Although there are definitive clinical differences of these entities, as mentioned above, it is the histopathologic features that can diagnostically separate the three. Wegener's granulomatosis is predominantly a necrotizing granulomatous infiltrate that has a polymorphous infiltrate of neutrophils, plasma cells, and histiocytes and is very distinct from the eosinophilic granulomas of CSS or the lymphocytic ones of LYG. Finally, all of the diseases respond differently to medications. Patients with Churg-Strauss syndrome for the most part respond well to high dosages of oral steroids and usually do not require therapy with immunosuppressive agents. Therapy with steroids alone is not adequate for the treatment of Wegener's granulomatosis, and the therapy is a combination of steroids with chemotherapeutic agents, cyclophosphamide being the agent of choice. The most difficult disease to treat in this review is LYG.(ABSTRACT TRUNCATED AT 250 WORDS)