Silicosis and coal worker's pneumoconiosis (CWP) are two relatively common occupational diseases. Silicosis is the most common pneumoconiosis in the United States. It results from inhalation of the particulate form of quartz or other crystalline forms of silicon dioxide with a diameter of less than 5 micrograms. CWP is the result of inhalation of carbon particles. Pathologic features of the two diseases differ, yet their radiologic features are identical. Simple pneumoconiosis is characterized by multiple small rounded opacities of 1 to 5 mm in diameter and with a bias for the upper lung zones. Complicated pneumoconiosis results from confluence of small opacities into large opacities that form conglomerate masses or progressive massive fibrosis.