We are developing an automated method for determining a number of parameters related to the size and shape of the heart and of the lungs in chest radiographs. In order to obtain standard patterns of the cardiac shadow as “gold standards,” four radiologists traced their best estimates of the entire contour of the heart, including the largely invisible inferior margin, on 11 radiographs. These contours were analyzed by Fourier transform, and the results were used as a guide to obtain a shift‐variant cosine function which was applied to the prediction of the cardiac contour by fitting a limited number of detected heart boundary points. These points were obtained from analysis of edge gradients in two orthogonal directions. A simple observer study indicated that the contours of the heart shadows computed for 60 chest radiographs were generally acceptable to radiologists for estimation of the size and area of the projected heart. We also detected the rib cage and the edges of the diaphragm, which enabled us to determine the projected thoracic area. From these results, we calculated the cardiothoracic ratio and other parameters, such as the ratio of the projected heart area to the projected thoracic area.