The coupling time of ventricular extrasystoles (VE) leading to 41 episodes of ventricular tachycardia (VT) in 35 patients with acute myocardial infarction (AMI) was studied regarding the measurement of the precocity index (R-R/QT) and vulnerability index (R-R x QT/R-R') described by other authors. A comparative group was studied in 50 VE of other patients with AMI but without VT. The majority of episodes (83%) of VT were originated by VE with a prematurity index of more than 1 (VE with coupling intervals longer than the QT duration of the previous cardiac cycle, R-R' > QT). The vulnerability index, with reported "dangerous" values of 1.1 to 1.4, was less than 1 in 92.6% of cases of VT and in 100% of VE without VT. A new index, delta QT, for the study of the abnormality of the QT interval respect to normal values (+/-.04") and its relation to the coupling time of the VE was also measured but did not show any difference among groups. In conclusion, none of the indexes were useful as predictor of ectopic beats that can produce VT in patients with AMI. We suggest that VE ocurring in patients with AMI should be suppressed disregarding the prematurity of diming of their appearance.