By using computer graphics we rotated the vector loop and three coordinate axes to find the viewpoint where the infarctional changes are maximally exposed and demonstrated the advantage of the "3-D VCG" over the conventional VCG by defining the quantitative "MI index." The orthogonal electrocardiogram recorded by the Frank lead system was digitally measured and processed by a microcomputer. The loops and axes were rotated about the X axis (X-rot) and the Y axis (Y-rot). The spatial vector loop and orthogonal coordinates can be presented as viewed from any spheric direction. Eight quadrants were illustrated with four colors and red circles. The subjects consisted of 30 patients with old anterior myocardial infarction (MI) and 15 patients with old inferior MI. We measured the area of "Bite" in anterior MI and superior displacement in inferior MI. The MI index was defined and averaged in 361 directions. In anterior MI, the maximum mean index was obtained when X-rot is +90 degrees and Y-rot -40 degrees, viewed from upward and leftward, whereas in inferior MI it was obtained when X-rot is -50 degrees and Y-rot -80 degrees, viewed from downward and leftward. These values were significantly higher than those in conventional VCG projections, substantiating superior diagnostic sensitivity of 3-D VCG.