ABSTRACTThrombolytic recanalization of the obstructed coronary lumen was studied in 32 patients receiving intracoronary streptokinase for 60 to 90 min during acute myocardial infarction. The process was viewed at high arteriographic magnification and was quantified with computer-assisted measurements from repeated single-plane views. The variability of the method for this application was 0. 15 to 0. 18 mm on minimum diameter estimates. Structural details were seen that are not commonly appreciated at conventional magnification. The recanalized lumen appears to form along an interface between the thrombus and the vessel wall, progressively enlarging its minimum arteriographic diameter to 0.65 0.24 mm (±+ 1 SD) at the end of the short-term infusion of streptokinase reflecting a final % stenosis of 77 10%. In nine infarct lesions found patent 5 + 3 weeks later, the recanalized lumen further improved an average of 0.34 mm in minimum diameter (p < .005) and 13% stenosis (p < .01). A thin film of contrast medium surrounding the obstructing thrombus faintly defined the boundaries of the original atherosclerotic lumen in all but two cases. The “original stenosis” measured 1.25 0.32 mm in minimum diameter and 56 14% stenosis when first visualized; it was unchanged throughout the course of infusion of streptokinase. In five patients catheterized 10 + 12 weeks before their infarction, the original stenosis averaged 1.15 0.22 mm in the preinfarct angiogram, as compared with 1.17 + 0.23 mm in its faintly defined form during thrombolytic therapy (p = NS). In 10 cases, this original lesion was less than a 50% stenosis, and in 21 cases less than 60%. These measurements permit an objective evaluation of the thrombolytic process; they demonstrate that mildto- moderate atherosclerotic coronary lesions are subject to acute thrombotic occlusion and that intracoronary streptokinase administered over 60 to 90 min only partially lyses the obstructing thrombus.