To determine the relation between spontaneous and induced ventricular arrhythmias, ambulatory electrocardiographic (Holter) monitoring and programmed electrical stimulation were performed in 48 adult patients with suspected life-threatening ventricular arrhythmias. Nine had no inducible arrhythmia, 11 demonstrated 1 to 2 beats of intraventricular reentry, 19 exhibited non-sustained ventricular tachycardia and 9 exhibited sustained ventricular tachycardia during electrophysiologic studies. Patients without arrhythmia inducibility had a high incidence of multiformity (56%) and bigeminy (44%), but a low incidence of either couplets (11%) or spontaneous ventricular tachycardia (11%) on Holter monitoring. An increasing incidence of all "complex" ectopic features was found with increasing degrees of ventricular inducibility. In patients with inducible sustained ventricular tachycardia, multiformity was present in 100%, bigeminy and couplets in 89% and spontaneous ventricular tachycardia in 78%. Premature ventricular complex frequency, couplet frequency and the repetition index (the ratio of couplets to premature ventricular complexes) were also found to be directly correlated with the degree of ventricular inducibility. Three quantitative arrhythmia variables were identified which predicted ventricular tachycardia inducibility. Seven (78%) of 9 patients with a mean premature ventricular complex frequency of 100 or more/1,000 normal beats, 11 (85%) of 13 with a mean couplet frequency of 1 or more/1,000 normal beats and 19 (83%) of 23 with a mean repetition index value of 15 or more/1,000 premature ventricular complexes proved to have inducible ventricular tachycardia.(ABSTRACT TRUNCATED AT 250 WORDS)