The sudden, often unheralded, onset of symptoms in acute myocardial infarction suggests that pre-existing coronary stenoses susceptible to acute thrombosis in the infarct-related artery may not necessarily have been severe. We investigated the severity of residual coronary stenoses after successful thrombolytic recanalization and the relationship to previous symptoms, collateral vessels and the extent of coronary artery disease in 60 consecutive patients at the time of presentation of their first acute myocardial infarction by performing quantitative coronary arteriography before, during and after intracoronary thrombolytic therapy. Recanalization was achieved in 48 (80%) patients with a residual stenosis of 58.1 +/- 10.8% (mean +/- ISD; range 33-82%) obstruction diameter and a minimum lumen calibre of 1.10 +/- 0.3 mm (range 0.39-1.95 mm). A residual stenosis of less than 60% obstruction diameter was present in 28 (47%) patients. When residual stenoses were mild, no acute collateral filling of the occluded artery was observed. After thrombolysis, residual infarct-related coronary stenoses in patients with their first acute myocardial infarction are not necessarily severely obstructive. This raises the problem of identifying which non-obstructive coronary stenoses are likely to occlude suddenly and why they do so.