A technique for automated detection of abnormal asymmetry in digital chest radiographs is being developed. Such a method could be used to prescreen chest radiographs to bring obviously abnormal cases to the immediate attention of a radiologist. In addition, this technique may be used to detect large-area abnormalities which may cause other, more lesion-specific computer algorithms to fail. Asymmetric abnormalities are detected by multiple stages of gray-level thresholding. Lung contours are determined, and after a centroid test is used to eliminate contours external to the lungs, the areas of remaining contours are calculated. The present scheme, applied to a database of 70 chest images, yielded a sensitivity of 91% and a specificity of 80%.