In 104 patients with various type of bundle branch block the three most known systems of orthogonal ECG leads (Frank; McFee-Parungao; Beswick-Jordan) were compared with 12 routine leads. All three orthogonal systems proved to possess good informative value, not inferior to that obtained by routine ECG. Examinations of 123 patients suffering from ischaemic heart disease with signs of diffuse myocardial changes with the aid of the Frank's system of orthogonal ECG confirmed the high informative value of the system in the detection of local disturbances of intraventricular conduction. Criteria of evaluation of orthogonal ECG tracings in the detection of left anterior, right, and left bundle branch block were elaborated. The high informative value and limited number of leads recommend the orthogonal ECG system for wide practical use.