25 patients who had undergone intracoronary thrombolysis (ICT) during acute myocardial infarction were studied an average of 9.8 months after the event in order to test the effect of the procedure on the incidence of complex ventricular tachyarrhythmias, late potentials determined by the signal-averaging technique, ejection fraction, and left ventricular asynergy. The relationships between these findings were tested. ICT was successful in 17 patients. There was a statistically significant relationship (p less than 0.05) between demonstrable akinesia and dyskinesia, late potentials, and complex ventricular tachyarrhythmias. Akinesia and dyskinesia and complex tachyarrhythmias were found more often in the 8 patients in whom ICT was unsuccessful than in the remaining 17 subjects (p less than 0.05). In the latter group no relationship was found between duration of occlusion and the incidence of asynergy, complex tachyarrhythmias, and late potentials; occlusion time was more than 200 min in 7 patients and less than 200 min in 10 patients. It is desirable to hold occlusion time below 200 min if there is no collateral circulation to the jeopardized myocardium.