Recent studies have shown that pain at rest in patients with unstable angina pectoris is often caused by transient reduction in regional myocardial perfusion. Coronary spasm has been implicated as a mechanism of this phenomenon. Recent reports have documented the occurrence of intracoronary thrombus in patients with unstable angina. Previous surveys have estimated a 6 to 12% frequency of intracoronary thrombus in this syndrome, but have not examined whether this incidence is related to how recent the angina at rest was. Angiograms of 119 patients with unstable angina who had rest pain within 14 days of angiography and 35 patients with stable angina were surveyed. Patients with unstable angina were subgrouped according to how recent angina at rest was at the time of angiography. Group I consisted of 44 patients in whom rest pain occurred within 24 hours before angiography. The 75 patients in group II had angina at rest between 1 and 14 days before angiography. Patients in group II had stable angina. The angiographic criterion for intracoronary thrombus was an intraluminal filling defect, surrounded by contrast medium on 3 sides, located just distal to or within a coronary stenosis, as assessed by each of 2 independent observers blinded to the nature of the anginal syndrome and its temporal proximity. Intracoronary thrombi were found in 44 of 119 patients with unstable angina (37%) and 0 of 35 patients with stable angina (p less than 0.00002). Intracoronary thrombi were found in 23 of 44 patients (52%) in group I and 21 of 75 (28%) in group II (p less than 0.008).(ABSTRACT TRUNCATED AT 250 WORDS)