We studied 19 patients diagnosed of chronic pericarditis and treated surgically with pericardiectomy, to evaluate the role of the pericardium and myocardium on the preoperative electrocardiogram. Before surgery, there was atrial fibrillation in 17 cases, the P axis was between 0 and +70 and pseudomitral morphology was found in 10 of the 12 cases with normal sinus rhythm. The mean voltage in the QRS complex was (13.8 mm), and the R wave in V5 8.6 mm. The T wave was negative in 17 patients. After surgery there was only a case of atrial fibrillation that converted to normal sinus rhythm; the P wave axis moved to the right between +30 and +150 (p less than 0.002), the pseudomitral morphology of the P changed to normal in 3 cases, the voltage of the QRS increased to 16.7 mm (p less than 0.003) and that of the R wave in V5 to 9.4 mm (p less than 0.001). The T wave became positive in only 3 cases. Because of the persistence of some alterations, we concluded that both components, pericardium and myocardium are important in the changes of the electrocardiogram of constrictive pericarditis.