Strong positive staining for BiP, ERAD-enhancing α-mannosidase-like proteins (EDEM), and XBP1 in epithelial cells from IPF lungs, expression of misfolded precursor protein for surfactant protein-C (pro-SP-C), mutant protein aggregates in A549 type II alveolar epithelial cells, and expression of p50ATF6 (i.e. a processed form of ATF6), ATF4, and CHOP in sporadic IPF lung samples support a correlation between ER stress and IPF. This evidence concerns the gene XBP1 and idiopathic interstitial pneumonia.