The coronary anatomy of 69 patients with unstable angina, subgrouped according to response to medical therapy, was investigated. All patients received oral treatment with nitrates, calcium antagonists, and beta-blocking agents. When combined oral treatment was not effective, an intravenous infusion of nitrates (10-100 micrograms/min) was subsequently administered. Coronary arteriography was performed within hours (14 +/- 9 h) from the last episode of chest pain in 28 patients refractory to medical treatment, while in 41 patients who became asymptomatic during medical therapy, angiography was performed after an observation period of several days (8 +/- 6 days). On angiography, the nonresponder group was characterized by a prevalence of eccentric and multiple lesions, and by a 46% incidence of thrombi (p less than 0.001). Recurrent symptoms requiring emergency bypass operation were common in this group. In patients responsive to medical treatment, a high percentage of concentric lesions (37%) and totally occluded (34%) coronary arteries was found (p less than 0.05). No infarcts and low rate of recurrent angina were noted in these patients during hospitalization. In conclusion, the finding of intracoronary thrombotic material and eccentric or multiple lesions can be an accurate markers of the active phase of the disease, while "silent" occlusion of the involved vessel may be accompanied by relief of symptoms during medical therapy.