The S1 S2 S3 pattern in the electrocardiogram has been variously defined. Some apply this term to all cases with an S wave in each standard lead, regardless of magnitude, while others use it to indicate situations where the prominent QRS deflection is an S wave in these leads. This latter application, which we prefer, is generally associated with marked right ventricular hypertension, although little has been published specifically about this pattern in children. We have reviewed 100 pediatric patients with electrocardiograms showing predominant S waves in each of the three standard leads. Ninety cases had congenital cardiac defects which were generally associated with manifestations of right ventricular hypertrophy and right ventricular hypertension. The pattern was most common in children with complete transposition of the great vessels and associated interventricular communications and in children having ventricular septal defects with pulmonary hypertension. In addition, in 10 cases without manifestations of cardiac disease this electrocardiographic pattern was demonstrated, although it was usually of an indeterminate axis type, and was associated with an otherwise normal electrocardiogram.