The purpose of this study was to quantify coronary morphology under medical therapy in patients with initially unstable angina pectoris (Propranolol, long-term nitrates). In 25 patients the extent of 69 coronary stenoses was exactly quantified with a vernier caliper (accuracy: 0.05 mm) in two successive coronary angiograms performed in each patient at approximately one year interval. The validity of the measuring technique was assessed in nine hearts in which 19 coronary stenoses were measured in the intravital coronary angiogram and then compared with the post mortem planimetric measurements of the same cross-sections after pressure fixation. This relation yielded a correlation coefficient of r = 0.87. In the patients with initially unstable angina the average degree of the stenoses in the three major coronary branches did not change significantly over the one year interval: In 27 stenoses of the right coronary artery the mean degree of obstruction averaged 79% in the initial angiogram and 84% in the second angiogram. In 26 stenoses of the left anterior descending artery the degree of obstruction was 78% and 77%, respectively and in 16 stenoses of the left circumflex artery 73% and 83%, respectively. Only 14 out of 69 stenoses (= 20%) showed a distinct progression of more than 20% area obstruction. All stenoses of 90% or more in the first angiogram (n = 6) progressed to complete obstruction within one year. In contrast, in five other stenoses we measured a regression in the degree of obstruction of more than 20%. We conclude from these quantitative measurements that the severity and the distribution of coronary lesions are similar in patients with stable or unstable angina pectoris. Coronary anatomy showed no significant change after one year of medical treatment in patients with unstable angina.