Churg-Strauss syndrome is a rare systemic vasculitis occurring in patients with asthma and blood eosinophilia. Lungs, skin, and nervous system are the most common sites of involvement, although many other organs are affected frequently. The diagnosis often is established from clinical findings or biopsy of extrapulmonary sites, and lung biopsy is performed infrequently. The classic pathologic findings in the lung include a combination of eosinophilic pneumonia, granulomatous inflammation, and vasculitis. All 3 features may not be present in every case, however, and diagnosis often requires careful correlation of the clinical and pathologic findings. The differential diagnosis in the lung includes diseases that are associated with eosinophil infiltrates or a combination of eosinophil infiltrates and granulomatous inflammation. Distinguishing these various diseases from Churg-Strauss syndrome is especially important, since many are more common than Churg-Strauss syndrome, and treatment is usually different.