The electrocardiogram (ECG) in patients with primary pulmonary hypertension who died suddenly (Group II, 12 patients) and those who died of right heart failure (Group III, 19 patients) were compared with those of long-term survivors (Group I, 10 patients). In P waves, AP had a tendency to deviate more to the right in Groups II and III than in Group I. P terminal force in lead V1 tended to show a greater absolute value, and the amplitude of P waves in lead II were higher in Groups II and III than in Group I. The AQRS in Groups II and III deviated significantly much more to the right than in Group I. In lead V1, S waves had a tendency to be deeper and the R/S ratio greater in Groups II and III than in Group I. In leads V5 and V6, R waves and the R/S ratio significantly lower in Groups II and III than in Group I. S waves in lead V6 tended to be deeper in Groups II and III than in Group I. In the ECG patterns, S1O3, inverted T waves in leads II, III and a VF and qR patterns in lead V1 tended to appear more often in Groups II and III than in Group I. In Groups II and III, there was T wave inversion beyond V3 to the left chest leads and significant differences between Groups II and III, and Group I.(ABSTRACT TRUNCATED AT 250 WORDS)