Exposure to synthetic fibers with employment in textile mills can be associated with an elevated risk of interstitial lung disease (ILD). A mechanism of injury has not been determined. ILD can follow exposures to inorganic fibers (e.g., asbestos) which are associated with a mobilization of iron and catalysis of an oxidative stress. We describe 2 patients with ILD associated with exposure to synthetic textile fibers who demonstrated carbon-based ferruginous bodies suggesting an in vivo accumulation of iron by synthetic fibers after deposition in the lung. These iron-laden bodies varied from perfectly linear fibers to almost particulate matter. Linear structures were irregularly interrupted by deposition of iron-abundant material. The capacity of these synthetic fibers to complex iron and generate an oxidative stress is confirmed in vitro.