Ventricular arrhythmias during thrombolysis for acute myocardial infarction and their relation to coronary artery patency were examined. Twenty-four-hour Holter monitoring was begun 3.1 +/- 0.2 hours after onset of pain in 40 patients (age 54 +/- 1.6 years; anterior infarction 42.5%) treated with streptokinase (42.5%) or recombinant tissue-type plasminogen activator (57.5%) (delay from pain 3.3 +/- 0.2 hours). A Marquette 8000 computer was used for Holter analysis. The infarct-related artery was considered as patent (72.5%) or nonpatent (27.5%) according to coronary angiography (delay from pain 26.7 +/- 2.5 hours; 60% < 24 hours). Ventricular arrhythmias were present in all patients. Tolerance was good (1 cardioversion for ventricular fibrillation). The incidence of accelerated idioventricular rhythm was not different between patients with a patent and nonpatent artery (90 vs 82%), nor for ventricular tachycardia (VT) (83 vs 73%). Coronary artery patency was associated with a 14-, 13- and 32-fold increase of ventricular premature complexes, VT and accelerated idioventricular rhythms, respectively. The increased incidence of sustained VT (patent 38%; nonpatent 0%; p < 0.05) and early (before the first 6 hours) accelerated idioventricular rhythm (patent 76%; nonpatent 18%; p < 0.01) associated with artery patency suggests that these arrhythmias may be noninvasive diagnostic criteria for reperfusion (sensitivity 38 vs 76%, and specificity 100 vs 82%). A positive correlation was found between the frequency of ventricular premature complexes and VT, and peak creatine kinase.