Two hundred and forty percutaneous transluminal coronary angioplasty procedures were performed in three centres over a two year period. Acute occlusion of the vessel undergoing angioplasty was seen on 20 (8%) occasions. The cause of occlusion was determined angiographically and in some cases confirmed at the time of emergency open heart surgery. The mechanism of coronary occlusion was arterial dissection in six cases, persisting coronary arterial spasm in seven, and coronary thrombosis in four. In three patients the mechanism could not be determined. Immediate reintroduction of a balloon dilatation catheter was attempted in 10 patients and resulted in restoration of adequate coronary flow in six. The remaining 14 patients underwent open heart surgery as an emergency procedure.