In the past, pleural disease has been uncommon, generally limited to infection derived from underlying pulmonary involvement or the result of local neoplastic invasion or hematogenous metastases. The deep, protected location of the lung's mesothelial surface provides insufficient defense against environmentally derived very fine biologically active inorganic particles, and a new set of abnormalities--pleural plaques, fibrosis, unique calcification, malignancy (mesothelioma), benign asbestotic effusion--have introduced problems of pathogenesis, diagnosis, management, and therapy. These changes are becoming frequent among individuals who were exposed to asbestos more than 20 years ago. Occupational exposure (direct and indirect), and in some cases environmental exposure (household contacts of asbestos-exposed workers and factory neighborhood residents), have been associated with higher prevalence of radiographically evident pleural abnormalities. What effects such changes will have on morbidity and mortality rates is incompletely understood.