Coronary angiography has many limitations for the assessment of coronary artery disease. Intracoronary ultrasound imaging may overcome some of these limitations by providing direct visualization of the luminal area and plaque morphologic features. Although the size of the currently available intracoronary ultrasound catheters precludes their use in many diseased coronary vessels, lesions in the relatively large vessels, such as the left main coronary artery, can be readily assessed. Intracoronary ultrasound imaging was performed in five patients in whom the status of the left main coronary artery was unclear after conventional coronary angiography. Qualitative assessment of atherosclerotic involvement and quantitative analysis of the absolute luminal area and the percentage of area of stenosis were performed. No complications were associated with the intracoronary ultrasound procedure. In all five patients, the ultrasound studies provided additional information on which a clinical decision could be made. Intracoronary ultrasound imaging is useful for assessing disease of the left main coronary artery in selected patients in whom current angiographic techniques have provided equivocal results.