This study compares the ability of intracoronary ultrasound (ICUS) to identify thrombus by means of actual criteria, with the histologic studies of tissues removed by directional atherectomy in patients treated previously with thrombolytic therapy. Coronary angiography and intravascular ultrasound imaging were performed before atherectomy in 34 patients who had received intravenous thrombolytic therapy for acute myocardial infarction a mean of 6 days before. The lesion morphology and the percentage of stenosis were defined on the angiogram. The ultrasound characteristics of the narrowing were described as intraluminal thrombus, mural thrombus, mixed plaque, and dense plaque. Thirty patients were studied. Thrombus was suspected in 8 patients on angiography. By ICUS, the presence of thrombus was predicted in 21 patients. Histologic studies of excised tissues found thrombus in 20 of the 30 patients. When ICUS was compared with histology, the true-positive rate was 80% and the false-positive rate was 50%; the true-negative rate was 50% and the false-negative rate was 20%. The correlation between observers was high. These observations suggest that ICUS may be useful in identifying fresh thrombus. The findings of this study help to confirm the criteria for diagnosing intraluminal thrombus by ICUS imaging.