We used angiography in a prospective study of the coronary circulation in patients with acute coronary insufficiency. Reversible ST-T changes during the acute illness corresponded anatomically with severely narrowed coronary arteries (80 to 95 per cent stenosis). Angiograms repeated four months later showed new complete occlusions in nine of 30 severely stenotic arteries. Eight of the new occlusions occurred in severely narrowed arteries previously correlated with regional ST-T changes. Six patients had myocardial infarctions, five of which corresponded with the site of a new occlusion. These results provide indirect evidence that the acute coronary-insufficiency syndrome commonly represents intermittent transient coronary-artery occlusion and a threat of new permanent occlusion of the same artery. Myocardial infarction in these patients appeared to occur as a complication of the new occlusion.