Slate is a metamorphic rock comprising silica, aluminum silicates, and small amounts of chlorite, hematite, magnetite, and various carbonates. In the United States slate is quarried in Virginia, Pennsylvania, New York, and Vermont. Workers are exposed during mining and processing of the slate and in crushing mills that prepare gravel. We have conducted detailed mineralogic and pathologic studies on the lungs of 12 workers who developed a pneumoconiosis while employed in the quarries of west central Vermont and adjacent areas of New York. Perivascular and peribronchial lesions accompanied by interstitial fibrosis and macules were scattered diffusely in the lungs. The lesions were associated with a variable number of silicotic nodules. Scanning electron microscopy combined with energy-dispersive x-ray spectrometry demonstrated aluminum and silicon-containing particles with variable cationic constituents and silicon alone in a pattern typical of free crystalline quartz. By x-ray diffraction analysis the majority of the mineral particulates were free crystalline quartz and muscovite, an aluminum silicate in the mica group of minerals. Slateworkers are exposed to respirable airborne dust that has the capacity to produce a pneumoconiosis that differs from classic silicosis.