ACO occurred within 40 min (mean 15.7 min) after PTCA in 22 patients and more than 12 hours in 2. The group with ACO had a significantly higher incidence of female (46% vs 23%), acute myocardial infarction (63% vs 35%), eccentric lesions (73% vs 28%), tortuous lesions (30% vs 4%) and coronary dissection or intraluminal haziness (89% vs 34%). Luminal narrowing before and after PTCA was significantly higher in ACO group than in control group (93% and 56% vs 87% and 23%). Repeat PTCA was performed in 17 patients and was successful in 13. Coronary bypass surgery was performed in 4 patients. Intracoronary urokinase was ineffective to ACO. In 3 autopsy cases dying 5, 14 and 17 days after PTCA, large extent of intimal tears and thrombus in the space of tears and the lumens were observed. These results suggest that coronary dissection chiefly contributes to ACO and coronary thrombosis is superimposed for a secondary event in most of cases with ACO.