Rationale: Interstitial lung diseases (ILDs) are associated with oxidative stress. Plasma biomarkers that are directly linked to oxidative stress responses in this disease have not been identified. Stable oxidation products of tyrosine residues in proteins may reflect the oxidative microenvironment in the lung or a systemic inflammatory state. Objectives: To determine if levels of protein tyrosine oxidation are elevated in plasma of patients with ILD compared with an age- and sex-matched healthy control cohort. Methods: Three tyrosine oxidation products (3-chlorotyrosine, 3-nitrotyrosine, and o,o9-dityrosine) were quantified by tandem mass spectrometry in cellular models, a mouse model of injuryinduced fibrosis, and in plasma of healthy control subjects and patients with ILD (n = 42 in each group). Measurements and Main Results: Plasma levels of 3-chlorotyrosine, 3-nitrotyrosine, and o,o9-dityrosine were markedly elevated in patients with ILD compared with control subjects with receiver operating characteristic curves separating these groups of 0.872, 0.893, and 0.997, respectively. In a murine model of lung fibrosis, levels of all three oxidative tyrosine modifications were increased in plasma and lung tissue. Cellular models support a critical role for a heme peroxidase and enzymatic sources of reactive oxygen species in the generation of these oxidized products. Conclusions: We demonstrate an increase in oxidized tyrosine moieties within proteins in the circulating plasma of patients with ILD. These data support the potential for development of oxidative stress-related biomarkers in early diagnosis, prognostication, and/or in evaluating responsiveness to emerging therapies for ILD.