Fifty-eight consecutive post-myocardial infarction (MI) patients undergoing surgery for left ventricular (LV) aneurysm were studied retrospectively to assess the frequency, characteristics, and complications of mural thrombi found at surgery, and to determine what effect chronic anticoagulation might have had upon the clinical course of these patients. Mural thrombi at surgery were found in 66% (38/58). Of these, only 10 (26%) were prospectively identified by LV angiography; conversely, LV angiography misidentified the presence of mural thrombi in 10% (2/20) who had no thrombus at surgery. Patients with mural thrombi were not different from those without thrombi in terms of time elapsed since their MI (28 ± 7 vs 24 ± 10 mos), LV end-diastolic pressure (LVEDP) (18 ± 2 vs 18 ± 2 mm Hg), ejection fraction (22 ± 1 vs 25 ± 2%), or angiographic scar size (33 ± 2 vs 29 ± 3%). Of 17 patients receiving long-term therapy with warfarin sodium preoperatively for anticoagulation, 9 had mural thrombi, whereas 8 did not (NS). There were only 2 patients in the total group (2/58 = 3%) with a preoperative event compatible with systemic arterial embolization - one of these was anticoagulated. Thus, in postmyocardial patients having LV aneurysmectomy; (1) the prevalence of mural thrombus is high but cannot be reliably identified prospectively by LV angiography or predicted by time since their MI, angiographic scar size, ejection fraction, or LVEDP; (2) the occurrence of preoperative systemic arterial embolization is very low; and (3) chronic anticoagulation has no apparent effect on the frequency of preoperative systemic arterial embolization or the prevalence of LV mural thrombus at surgery.