Pulmonary alveolar proteinosis mimicking idiopathic pulmonary fibrosis. 
Pulmonary alveolar proteinosis (PAP) is a rare disorder of unknown etiology.  While most biopsies from patients with PAP disclose a prominent positive periodic acid-Schiff, diastase-resistant intra-alveolar exudate, and minimal interstitial thickening, interstitial fibrosis and honeycombing can be the predominant histologic findings.  We recently identified two patients with PAP who initially were thought to have idiopathic pulmonary fibrosis, but whose biopsies ultimately disclosed PAP with considerable interstitial fibrosis.  Because the clinical and radiographic presentation of PAP can overlap with other interstitial lung disorders, it should be considered in the differential diagnosis of unexplained idiopathic interstitial pneumonias in young patients.