The effect of deferring immediate coronary artery bypass was evaluated in two groups of similar patients having successful direct coronary artery thrombolysis with streptokinase in the treatment of evolving myocardial infarction. Within 6 hours of onset of myocardial infarction, 140 patients underwent immediate cardiac catheterization and infusion of intracoronary streptokinase up to 500,000 units. Of those patients having restoration of orthograde coronary blood flow coupled with immediate evidence of myocardial salvage, 31 patients (group I) had immediate coronary artery bypass and 34 patients (group II) had coronary artery bypass deferred. Group I had no hemorrhagic, hemodynamic, or new cardiac complications. There were no deaths, reinfarction, recurrence of angina, or loss of salvaged myocardium at restudy. In group II, 11 of 34 patients had early reinfarction (p less than 0.01 vs. group I), 16 of 34 patients had recurrent angina (p less than 0.01 vs. group I) and two deaths occurred from cardiac causes. Subsequent coronary bypass was needed in 16 patients. All restudied reinfarction patients lost restored myocardium. We concluded that immediate coronary artery bypass is safe after acute myocardial infarction and coronary artery thrombolysis with streptokinase, delay of coronary artery bypass leads to an unacceptable incidence of reinfarction and recurrent severe angina, and early coronary artery bypass is needed to ensure success of thrombolysis.