This series included 26 patients with acute myocardial infarction (15 patients) or unstable angina (11 patients), with thrombotic lesions identified by angiography and treated with intravenous heparin (mean 8 +/- 3 days, aPTT 1.5-2.5 times baseline) and aspirin (250 mg/day). A quantitative angiographic analysis was performed, and comparison of the lesion morphology before and after the antithrombotic treatment was conducted, using a computer-based analysis of the lesion contour. On average, minimal lumen diameter increased by 15.6% (1.15 +/- 0.5mm to 1.33 +/- 0.5mm, P = 0.02), percent diameter stenosis decreased by 10.7% (60.0 +/- 18% to 53.6 +/- 19%, P = 0.009), percent area stenosis decreased by 6.5% (80.0 +/- 14% to 74.8 +/- 16%, P = 0.05), and TIMI flow increased by 8% (2.5 +/- 0.9 to 2.7 +/- 0.7, P = 0.02). A significant improvement in the lesion morphology was more often observed in the right coronary artery (n = 9), than the left coronary artery (n = 16). We conclude that heparin and aspirin therapy improves the angiographic morphology and TIMI flow of thrombotic lesions in patients sustaining the acute coronary syndrome. This improvement was primarily found when the lesion was located in the right coronary artery.