Diffuse lung diseases (DLD) are a heterogeneous group of diseases with different etiopathogenesis, clinical course, and prognosis. It has been demonstrated that oxidative stress can contribute to the pathogenesis of these diseases. In the present study we measured carbonylated protein concentrations in the BAL of patients with sarcoidosis, pulmonary fibrosis associated with systemic sclerosis, idiopathic pulmonary fibrosis, and for the first time in patients with chronic eosinophilic pneumonia and extrinsic allergic alveolitis. Our aim was to further investigate oxidation products in diffuse lung diseases. Oxidatively modified protein concentrations were increased in the BAL of patients than in that of controls (0.22 nmol/mg protein vs 0.05 nmol/mg protein; p < 0.001) and in each group of disease versus controls, suggesting that proteins that have become dysfunctional by oxidation could play a role in the pathogenesis of diffuse lung diseases. Further studies in a greater number of patients are needed to understand the contribution of oxidatively modified proteins to the pathogenesis of DLD and, in particular, to the development of extrinsic allergic alveolitis where the highest levels of carbonylated proteins were found.