A review of the relevant literature provides evidence that cigarette smoking causes microscopic diffuse interstitial pulmonary fibrosis in humans at autopsy. The association shows a dose-response relationship and the fibrosis is often severe. Experimental animal studies have confirmed these observations when the exposure to inhaled smoke was prolonged or heavy. Six published and one unpublished cross-sectional chest x-ray surveys of adults have shown low profusion of small irregular opacities related to smoking with a dose-response relationship when this has been studied. Although information on radiographic-pathologic correlation is deficient, these studies support the hypothesis that smoking causes diffuse interstitial pulmonary fibrosis which is radiologically visible with low profusion in low prevalence. The x-ray manifestations may be confused with early pneumoconiosis such as asbestosis.