Late potentials (LPs) were studied using the signal averaging technique in 80 patients with myocardial infarction (MI), idiopathic cardiomyopathy (CM) and idiopathic ventricular tachycardia (IVT). In MI, LP duration was 28.4 +/- 12.6 ms in the sustained VT group (I; 8 cases); 18.6 +/- 9.0 ms in the non-sustained VT group (II; 11 cases); and 14.4 +/- 8.6 ms in the non-VT group (III; 21 cases); (p less than 0.05 in I vs II, p less than 0.01 in I vs III and not significant in II vs III). In CM, it was 33.7 +/- 13.0 ms in group I (6 cases); 20.1 +/- 5.9 ms in group II (12 cases); and 7.1 +/- 9.2 ms in group III (14 cases); (p less than 0.01 in I vs II, I vs III and II vs III). The LP duration in IVT (8 cases) was 15.6 +/- 10.4 ms, which was significantly shorter than that of group I in MI and CM (p less than 0.05 vs MI and p less than 0.01 vs CM). Late potential duration was also compared between a pacing-inducible VT group and a non-inducible VT group. The mean value of LP duration in the inducible VT group of MI was significantly longer than that in the non-inducible group (27.8 +/- 3.9 ms in 4 cases vs 17.3 +/- 2.5 ms in 4 cases, p less than 0.05). However, there was no significant difference in LP duration between the inducible and non-inducible groups of CM (22.0 +/- 11.0 ms in 5 cases vs 22.2 +/- 13.6 ms in 5 cases).(ABSTRACT TRUNCATED AT 250 WORDS)