The authors present electrocardiographic body surface potential maps (BSPMs) of 11 patients with hypoplastic right ventricle (HRV) of three types: type I, HRV with pulmonary atresia; type II, HRV with tricuspid atresia; and type III, HRV with tricuspid artesia and transposition of the great arteries. The BSPMs of all 11 patients demonstrated evidence for epicardial right ventricular breakthrough, indicating conduction through an intact right bundle branch and Purkinje system. Nonetheless, the BSPMs strongly suggested profound morphological, probably embryological, differences among the right ventricles of the three groups. The four patients with type I HRV had no evidence for conduction abnormality. The five patients with type II, HRV however, had very marked conduction abnormality. In four of these five, the standard ECG and VCG had initial forces suggesting left lateral wall myocardial infarction. The BSPMs showed no evidence for infarction but demonstrated very complicated slow initial activation, explaining why the initial QRS vector was to the right and posterior before extending leftward. In addition, in all five the initial positive potentials were unusually inferior and the initial negative potentials unusually superior. After the evidence for epicardial right ventricular breakthrough, the positive and negative potentials rapidly changed positions so that the positive potentials were unusually superior and the negative potentials unusually inferior, consistent with the BSPM of endocardial cushion defects. In four of these five there was marked delay of total ventricular activation time. Of the two patients with type III HRV, one had an initial QRS similar to that of type II. Neither had rapid change of inferior and superior positive and negative potentials after right ventricular breakthrough, and both had intraventricular slowing, one with partial left bundle branch block.