The Thrombolysis in Myocardial Infarction II A Study investigated whether immediate cardiac catheterization with percutaneous transluminal coronary angioplasty (PTCA), when appropriate, would confer an advantage over the same procedures performed 18 to 48 hours later. All patients were treated with intravenous recombinant tissue-type plasminogen activator within four hours of the onset of acute myocardial infarction. Percutaneous transluminal coronary angioplasty of the infarct-related artery was attempted in 72% of the 195 patients assigned to immediate PTCA; 84% of the attempts were judged to have shown improvement. Percutaneous transluminal coronary angioplasty was attempted in 55% of the 194 patients assigned to 18- to 48-hour PTCA; 93% of the attempts were judged to have shown improvement. No differences between the two PTCA groups were observed for ejection fraction (primary end point), measured by contrast ventriculography predischarge (50.3% in the immediate and 49.0% in the delayed PTCA groups). Immediate catheterization/angioplasty was associated with increased frequency of bleeding and coronary artery bypass surgery. These findings indicate that immediate performance of coronary arteriography and PTCA compared with delaying these procedures for 18 to 48 hours provides no advantage and may be harmful.