The histologic findings, natural history, and response to treatment were investigated in 62 patients with pulmonary angiitis andgranulomatosis seen over a 23 year period. Three histologic types were recognized: lymphocyte depleted angiitis and granulomatosis, 24 cases (39 per cent); benign lymphocytic angiitis and granulomatosis, 14 cases (22 per cent); and malignant lymphoproiferative angiitis and granulomatosis, 24 cases (39 per cent). Lymphocyte depleted angiitis and granulomatosis corresponds to Wegener's granulomatosis of either the "limited" or "classic" variant. The lesions frequently show extensive liquefactive necrosis. Eosinophils may be abundant, but lymphocytes and plasma cells are relatively scarce. Histiocytic proliferation with the occasional presence of sarcoid-like granulomas was observed only in cases with no evidence of glomerular involvement. Cyclophosphamide is highly effective regardless of whether extrapulmonary manifestations are present. Benign lymphocytic angiitis and granulomatosis is characterized by dense benign infiltrates of lymphocytes, plasma cells, and histiocytes. Necrosis may be absent or minimal; the angiitis is usually mild to moderate. Only one of 14 patients (7 per cent) had extrapulmonary involvement (skin). Chlorambucil is highly effective, sometimes resulting in dramatic cures. Malignant lymphoproliferative angiitis and granulomatosis corresponds to lymphomatoid granulomatosis. Its lesions show extensive coagulative necrosis, a highly atypical lymphoreticular infiltrate with frequent mitoses, and angioinvasion. Extrapulmonary involvement is frequent (83 per cent), particularly in the skin (46 per cent) and central nervous system (33 per cent). The mortality is high: 65 per cent of the patients were dead within the first year of disease. Although cytotoxic drugs were of no value, corticosteroids were associated with prolonged remissions in three young patients.