The electrocardiographic and echocardiographic (M-mode) data were analyzed in 29 patients affected by mitral or combined mitral and aortic valve disease and with hemodynamically documented biventricular overload. No electrocardiographic parameter significantly correlated with the left ventricular internal dimension at end diastole (LVIDd), the left posterior wall (LVPW) thickness and the left ventricular mass (LV mass). A significant correlation was observed between the R/S ratio in V1 and V2, the rV1 and either the end-diastolic right ventricular internal dimension (RVIDs) or the pulmonary artery systolic pressure (PASP). The R/S ratio greater than or equal to 1 in V1 was the most sensitive among the conventional electrocardiographic criteria of right ventricular enlargement. Three groups of patients were selected on the basis of RVIDd and LV mass: Group A included nine patients with right ventricular dilatation and normal LV mass; Group B included ten patients without right ventricular dilatation and with increased LV mass; Group C included ten patients without right ventricular dilatation and with normal LV mass. The R/S ratio in V1 was significantly greater in patients in group A than in those in groups B or C. No electrocardiographic parameter was found to be significantly different between groups B and C. When only LV mass was considered (independently from RVId and PASP), no electrocardiographic parameter differentiated patients with LV mass greater than 203 g from those with LV mass less than 203 g. We conclude that in patients with biventricular overload secondary to acquired valvular disease: 1) the electrocardiographic diagnosis of left ventricular enlargement is unreliable; and 2) the R/S ratio in V1 is the most sensitive parameter to predict right ventricular enlargement or severe pulmonary hypertension.