The value of the electrocardiogram and vectorcardiogram for the diagnosis of congenital heart diseases with VSD and pulmonary obstruction is illustrated. Three types of electrocardiographic pattern are encountered: 1) pressure overload to the right ventricle (Tetralogy of Fallot, DORV, TGA); 2) left ventricular hypertrophy (TA, UH with outlet chamber to the right); 3) Inversion of septal activation (Correct TGA, UH with outlet chamber to the left). In the diagnosis of the anomalies of the first group, absence of P wave alteration, downward and rightward QRSA, lack of s wave in AVF and terminal forces of the QRS loop under the O point suggest Tetralogy of Fallot. In DORV and TGA the QRSA may be directed upward to the right or to the left and P wave may show overload pattern of the right ventricle. Moreover, in DORV conduction delays on the right bundle branch and/or increased voltage of the R wave in V1 may be observed. In diagnosis between TA and UH with outlet chamber to the right, a leftward displacement of QRSA, an overload pattern of RA and absence of clockwise rotation of the QRS loop on the H plane suggest the former disease. These electrocardiographic data, as well as pathological considerations, made us keep these two form distinct from a clinical point of view. Finally, recording of LV potentials in V1 or to its right with AV conduction abnormalities are typical of corrected TGA; in UH with outlet chamber to the left no conduction defects are observed. In UH with outlet chamber anterior o slightly to the left, ECG pattern is rS on all the praecordial leads and a left posterior hemiblock is present in 40% of the cases of second type.