There has been much controversy over the mechanism of successful percutaneous transluminal coronary angioplasty (PTCA). To examine clinical and angiographic factors that might be related to a successful PTCA, we assessed 224 branches treated with emergent or elective angioplasty in evolving acute myocardial infarction (AMI) and angina pectoris (AP). The patients were divided into three groups; group 1 (G1): AP (n = 113), group 2 (G2): AMI with complete obstruction of infarct-related artery (IRA) (n = 79), group 3 (G3): AMI with incomplete obstruction of IRA (n = 32). The morphology of stenotic lesion was classified into smooth type and irregular type. The former shows concentric smooth border, the latter shows eccentric irregular border and multiple irregular border according to the Ambrose classification. Regarding the severity of the stenosis immediately after successful PTCA, there was no significant differences between G2 and G3, however in G1 it was significantly higher than in other groups (G1 vs G2 vs G3, 56% vs 81% vs 78%, p less than 0.001). Irregular type at stenotic lesions before PTCA were present in 72 of 113 branches in G1 vs 60 of 69 in G2 and G3. Regarding the incidence of acute coronary obstruction during PTCA, there was no significant differences between G1 and G2. However, in G3 it was significantly higher than in other groups (G1 vs G2 vs G3, 8% vs 9% vs 38%, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)