To find electrocardiographic criteria for ventricular septal infarction, two series of ECGs were studied, all without fascicular/bundle branch block and complete heart block. One series consisted of the ECGs recorded at thallium-201 scintigraphy in 49 patients 2-3 weeks after an acute myocardial infarction (AMI): in this series the 14 patients with a defect in the septal wall of the left anterior oblique view were compared with the 35 without. The other series consisted of the last premortal ECGs in 20 AMI patients with and in seven without a septal involvement of the infarct at autopsy. The best criterion from earlier literature ws absence of a q wave in lead V6, showing a sensitivity of 53% in the combined scintigraphy and autopsy series. The predictive value of a positive test was 75%. The very first QRS vector in the frontal plane tended to discriminate better than absence of a q in lead V6. With a similar predictive value, 71%, the sensitivity of a deviating vector (+ 120 degrees to -60 degrees and -120 degrees to -180 degrees) was 65% in the combined scintigraphy and autopsy groups. Most of the patients with a deviating vector showed this on admission. The clinical importance of an early diagnosis of septal involvement in AMI remains to be settled.