Scanty irregular opacities are not uncommonly observed on the chest roentgenogram in the absence of interstitial fibrosis of the lungs. In such circumstances the irregular opacities, when present, tend to be relatively scanty and seldom, if ever, exceed an ILO category of 1/1. They are found in association with cigarette smoking, especially when emphysema is also present. The development of irregular opacities is also related to exposure to various mineral and other dusts, and although their prevalence increases with cumulative dust exposure, in general the type of dust, whether fibrogenic or relatively inert, seems to be of little moment. The presence of irregular opacities remains a troublesome confounding factor in epidemiologic studies of both dust-exposed and nonexposed populations. The morbid anatomic changes that occur in the lungs of nondust-exposed workers and which are responsible for the development of irregular opacities in the chest roentgenogram remain unknown.