Pulmonary vasculitis is usually caused by one of three disorders: (1) Wegener's granulomatosis (WG); (2) Churg-Strauss syndrome (CSS), or allergic angiitis and granulomatosis; or (3) a nonspecific small vessel systemic necrotizing vasculitis (SNV), or microscopic polyarteritis. WG, the most common cause of lung vasculitis, has features of a granulomatous vasculitis of the upper airway and lung and widespread small vessel vasculitis involving the kidneys and other organs. The features of pulmonary WG overlap with those of malignancy and infectious granulomatous lung disease; accurate diagnosis generally requires open lung biopsy. CSS is defined by the triad of asthma, eosinophilia, and systemic vasculitis. Easily accessible tissues should be biopsied, but the clinical features are so distinctive that tissue biopsy is not invariably required for diagnosis. CSS must be differentiated from other diseases that cause pulmonary infiltrates with eosinophilia, including infections. Nonspecific SNV causes diffuse alveolar hemorrhage due to pulmonary capillaritis. Concomitant segmental necrotizing glomerulonephritis is almost always present. Diagnosis is made by renal biopsy, compatible extrarenal features, exclusion of nonimmune causes of lung hemorrhage, and exclusion of WG to the extent possible.