Image processing technique is one of advantages associated with digital radiographic systems. Unsharp mask filtering has been investigated systematically, producing useful results. We investigated the effect of digital image processing with Wallis filter on a digitized chest radiograph. Wallis filter is similar to unsharp mask filter, except that Wallis filter can enhance local image contrast, depending on the standard deviation (SD) of the local image data. The high-frequency content of the processed chest radiograph is relatively high in small SD areas, and low in large SD areas. Therefore, the contrast of mediastinal area in the processed chest image was more enhanced than that of lung areas.