To describe the coronary arteriographic findings during early transmural myocardial infarction and to define the prevalence of coronary thrombosis by arteriography, we performed coronary arteriography and left ventriculography within 24 hours from symptom onset of transmural myocardial infarction in 517 patients. The coronary arteriographic findings included total and nontotal coronary occlusion with and without coronary thrombosis. Coronary spasm or severe stenosis in the affected vessel were discovered in a minority of patients. The coronary arteriographic characteristics of thrombus included persistent staining of intraluminal material by the contrast material, local retention of the contrast agent in the involved coronary artery, and intracoronary filling defect occurring mostly in nontotal occlusion. Of the 517 patients, 368 were studied within 6 hours of symptom onset, 85 within 6-12 hours and 64 within 12-24 hours. There was a systematic difference between total coronary occlusion and coronary thrombosis in each group. This was determined by arteriographic findings (judged positive or negative) relative to surgical findings. Both total coronary occlusion and coronary thrombosis were more prevalent in the early treatment (within 6 hours from symptom onset) group. Total occlusion and thrombosis declined significantly in the 6-12- and 12-24-hour groups. These data suggest that thrombus is encountered by arteriography and confirmed by surgical exploration within the first 6 hours from symptom onset of transmural infarction in approximately 80% of patients. Coronary thrombosis by coronary arteriography decreases in parallel with total coronary occlusion during the first 24 hours after acute transmural infarction, suggesting that coronary spasm or thrombus formation with subsequent recanalization are important in the evolution of transmural infarction. Despite many factors involved in the pathogenesis of transmural myocardial infarction, coronary thrombosis appears to be the final common pathway converting chronic coronary disease to acute myocardial infarction in the majority of patients.