Eosinophilic pneumonia-like areas in idiopathic usual interstitial pneumonia

Usual interstitial pneumonia is the most common idiopathic chronic interstitial pneumonia, characterized by a temporally heterogenous pattern of interstitial injury with interstitial mononuclear infiltrates, septal fibromyxoid nodules, and parenchymal scarring. This report details the presence of focal eosinophilic pneumonia in six cases of usual interstitial pneumonia in the absence of known causes of this reaction. The relationship of eosinophilic infiltrates in usual interstitial pneumonia with regard to pathogenesis, differential diagnosis, and prognosis is discussed.