Ferruginous bodies are frequently observed in histologic sections of lung from individuals with occupational or environmental exposure to asbestos and other mineral dusts. Analysis of large numbers of such ferruginous bodies has demonstrated that the types with asbestos cores can be differentiated by light microscopy from several chemically and morphologically distinct classes of nonasbestos ferruginous bodies, including those formed on sheet silicates (talc and mica), carbon, rutile, and fly ash. Light-microscopic screening of lung tissue sections for ferruginous bodies is a reliable, convenient, and economical method for documenting exposure to a variety of mineral dusts and can assist in determining the etiology of pneumoconiotic lesions. Because tissue sections are relatively insensitive detectors of particles, the finding of ferruginous bodies in section implies heavy dust exposure.