A novel method of estimating the computational errors in the shapes and the transverse areas of coronary arterial trees from biplane angiograms is described. Using an elliptical model, the intensity distributions of an ellipse in any image plane which is perpendicular to the plane of an arterial cross section are calculated. Given intensity distributions of ellipses in any two such image planes, one can show from their quadratic expressions that they have the same area. Computer-generated data for elliptical cross-sections at different viewing angles and sampling intervals are analyzed. The absolute percentage errors in estimating the lengths of the major amd minor axes and the rotation angle of an ellipse are computed from the computer-generated data and compared with the corresponding values obtained from biplane angiograms at a fixed viewing angle.