In order to establish the natural evolution of unstable angina under medical treatment and to determine the possible benefits of revascularization surgery, 113 patients were studied; 51 received medical treatment (24 with intermediate syndrome and 27 with progressive angina), and 62 others received surgical treatment (28 with intermediate syndrome and 34 with progressive angina). After a mean follow-up of 32 months, the mortality in the medically treated groups was 46 percent (11/24) with intermediate syndrome and 7 percent (2/27) with progressive angina (P less than 0.005), and the incidence of myocardial infarction was 38 percent (9/24) and 7 percent (2/27), respectively (P less than 0.01). Moreover, in comparing cases treated medically or surgically, the mortality was as follows: intermediate syndrome treated medically, 46 percent (11/24) vs 11 percent (3/28) in those treated surgically (P less than 0.005); and progressive angina treated medically, 7 percent (2/27) vs 9 percent (3/34) in those treated surgically (P = 0.85). The incidence of myocardial infarction was as follows: intermediate syndrome treated medically, 38 percent (9/24) vs 14 percent (4/28) in those treated surgically (P less than or equal to 0.056); and progressive angina treated medically, 7 percent (2/27) vs 12 percent (4/34) in those treated surgically (P greater than 0.55).