As previously reported in acute presentations of unstable angina, an identifiable characteristic coronary artery lesion has been found in about 70% of cases at coronary arteriography. This takes the form of an eccentrically placed convex stenosis with a narrow neck due to one or more overhanging edges or irregular, scalloped borders, or both. To study the evolution of lesions responsible for unstable angina, coronary artery anatomy and morphology on angiography were evaluated in patients with stable angina progressing to unstable angina. Group I comprised 25 patients with a history of stable angina who were restudied after an acute episode of unstable angina and Group II comprised 21 patients with little or no change in symptoms between catheterizations. Progression of coronary disease occurred in 19 (76%) of 25 patients in Group I compared with 7 (33%) of 21 in Group II (p less than 0.001). Of the 25 lesions with progression in Group I, 17 progressed to less than 100% and 8 to 100% occlusion. Eighteen of these 25 lesions in Group I were previously insignificant (less than 50% occlusion on the first catheterization). In contrast, of the eight lesions with disease progression in Group II, only two were previously insignificant while six showed at least 50% occlusion on the initial study. The eccentric lesion was seen in 71% of all lesions with progression to less than 100% occlusion in Group I, but it was not seen in any Group II vessel with progression.(ABSTRACT TRUNCATED AT 250 WORDS)