Non q wave myocardial infarction has been attributed to occlusion of a vessel with no ECG representation, early reperfusion of the occluded artery or occlusion of a vessel with generous collateral flow. The coronary arteriography of 84 patients with non Q wave myocardial infarction performed at 16 + 17 (SD) days after infarction was analyzed. Main left lesion was found in 6 (17%), single vessel disease in 30 (36%), two vessel disease in 18 (24%) and 3 vessel disease in 16 (19%). The "culprit" vessel had a critical residual lesion in 38 patients (45%): 22 affected the left anterior descending artery, 10 the circumflex, and 5 the right coronary artery. No residual lesion was found in 10 patients (12%). An occluded artery was found in 32 patients (38%): circumflex in 20, right coronary artery in 9 and left anterior descending in 3 (p < 0.01). Significant collateral flow to the occluded vessel was present in 41% of cases. The ST segment was analyzed in 82 patients. Depression of ST was found in 29 (35%), elevation in 22 (27%), negative T waves in 17 (21%) and minimal alterations in 17%. There was no correlation between ST levels and coronary occlusion of the culprit artery. Depression of ST was more commonly (p < 0.01) associated with severe coronary artery disease (main left or 3 vessel disease), which may be related to the poorer prognosis in these cases.