To assess possible clinical and angiographic factors associated with acute coronary events following PTCA, we performed quantitative angiography in 168 consecutive patients who had undergone successful angioplasty in a native vessel (94 for stable angina, 74 for unstable angina), and who were restudied (24 +/- 15 weeks; range 4 to 52) because of recurrent anginal symptoms. Of the 168 patients, 38 (Group 1) were restudied because the pattern of angina was aggressive (unstable angina in 31, myocardial infarction in 7) and 130 because of effort-related angina (Group 2). the two patient groups were well matched for extent of initial disease but patients in Group 1 were younger (P=0.03). PTCA for unstable angina was originally performed more frequently in Group 1 than in Group 2 (27 of 38 patients (71% vs 47 of 130 patients (36%), P=0.0004). Disease progression in non-dilated segments occurred in 10 patients (26%) in Group 1 compared with eight (6%) in Group 2 (P=0.0004). Disease progression in non-dilated segments occurred in nine patients (24%) in Group 1 and in Group 2 (P=0.0004). Disease progression in non-dilated segments occurred in nine patients (24%) in Group 1 and in 10 (8%) in Group 2 (P=0.0006). Our conclusion is that patients who require re-investigation as a result of angina which has become aggressive following PTCA are usually those who originally underwent PTCA for unstable angina. These patients have a higher incidence of occlusive restenosis or disease progression.