We have evaluated the feasibility of percutaneous transluminal coronary angioscopy for detecting intraluminal pathological changes as a diagnostic tool, and investigated the pathogenesis of two acute coronary disorders, acute myocardial infarction and unstable angina. Twelve patients with an acute coronary disorder and 20 patients who underwent percutaneous transluminal coronary angioplasty were selected for this comparison between the diagnostic accuracy of angioscopy and arteriography. One hundred and thirty patients were investigated by angioscopy as follows in order to investigate the pathogenesis of their acute coronary disorders: 22 within 8 h of onset acute myocardial infarction; 28 from one day to 2 months since onset recent myocardial infarction; 37 with an old myocardial infarction; 26 with unstable angina; and 23 with stable angina. Our results have indicated thrombi were detected more frequently by angioscopy than by arteriography (p less than 0.01). Also, thrombi, intimal irregularities, and xanthomatous atheromas were observed more frequently in patients with acute myocardial infarction, recent myocardial infarction, and unstable angina. It is concluded that coronary angioscopy is a much more sensitive method for detecting intraluminal changes, and that a thrombus overlying a rupture in the lining of plaque plays a major role in an acute coronary disorders, and that the fragile, lipid-rich gruel atheroma may precede its rupture.